diff --git a/accessible/generic/HyperTextAccessible.cpp b/accessible/generic/HyperTextAccessible.cpp
index 563ac6fe001..e344599ecde 100644
--- a/accessible/generic/HyperTextAccessible.cpp
+++ b/accessible/generic/HyperTextAccessible.cpp
@@ -1466,7 +1466,7 @@ HyperTextAccessible::CaretLineNumber()
       break;
 
     // Add lines for the sibling frames before the caret
-    nsIFrame *sibling = parentFrame->GetFirstPrincipalChild();
+    nsIFrame *sibling = parentFrame->PrincipalChildList().FirstChild();
     while (sibling && sibling != caretFrame) {
       nsAutoLineIterator lineIterForSibling = sibling->GetLineIterator();
       if (lineIterForSibling) {
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 1e18ac0572b..3c13f6f0fec 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -6917,7 +6917,7 @@ nsContentUtils::GetAdjustedOffsetInTextControl(nsIFrame* aOffsetFrame,
   // has the text frames (containing the content) as its children. This will
   // be the case if we click to the right of any of the text frames, or at the
   // bottom of the text area.
-  nsIFrame* firstChild = aOffsetFrame->GetFirstPrincipalChild();
+  nsIFrame* firstChild = aOffsetFrame->PrincipalChildList().FirstChild();
   if (firstChild) {
     // In this case, the passed-in offset is incorrect, and we want the length
     // of the entire content in the text control frame.
@@ -6930,7 +6930,7 @@ nsContentUtils::GetAdjustedOffsetInTextControl(nsIFrame* aOffsetFrame,
     // frame. Our offset should therefore be the length of the first child of
     // our parent.
     int32_t aOutOffset =
-      aOffsetFrame->GetParent()->GetFirstPrincipalChild()->GetContent()->Length();
+      aOffsetFrame->GetParent()->PrincipalChildList().FirstChild()->GetContent()->Length();
     return aOutOffset;
   }
 
diff --git a/dom/base/nsDocumentEncoder.cpp b/dom/base/nsDocumentEncoder.cpp
index 2e45a3043cc..55c972cc017 100644
--- a/dom/base/nsDocumentEncoder.cpp
+++ b/dom/base/nsDocumentEncoder.cpp
@@ -331,7 +331,7 @@ LineHasNonEmptyContentWorker(nsIFrame* aFrame)
   // Look for non-empty frames, but ignore inline and br frames.
   // For inline frames, descend into the children, if any.
   if (aFrame->GetType() == nsGkAtoms::inlineFrame) {
-    nsIFrame* child = aFrame->GetFirstPrincipalChild();
+    nsIFrame* child = aFrame->PrincipalChildList().FirstChild();
     while (child) {
       if (LineHasNonEmptyContentWorker(child)) {
         return true;
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index cad3985d5a3..f4c30e0e87b 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -9885,7 +9885,7 @@ static nsCanvasFrame* FindCanvasFrame(nsIFrame* aFrame)
         return canvasFrame;
     }
 
-    nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+    nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
     while (kid) {
         canvasFrame = FindCanvasFrame(kid);
         if (canvasFrame) {
diff --git a/dom/base/nsRange.cpp b/dom/base/nsRange.cpp
index 688f927a9cd..090895092db 100644
--- a/dom/base/nsRange.cpp
+++ b/dom/base/nsRange.cpp
@@ -3335,7 +3335,7 @@ IsLastNonemptyRowGroupOfTable(nsIFrame* aFrame)
   }
   for (nsIFrame* c = aFrame; c; c = c->GetNextContinuation()) {
     for (nsIFrame* next = c->GetNextSibling(); next; next = next->GetNextSibling()) {
-      if (next->GetFirstPrincipalChild()) {
+      if (next->PrincipalChildList().FirstChild()) {
         return false;
       }
     }
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 0de79536e4d..42f95ff7d9b 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -2917,7 +2917,7 @@ HTMLInputElement::Focus(ErrorResult& aError)
   // tab to the next one.
   nsIFrame* frame = GetPrimaryFrame();
   if (frame) {
-    for (nsIFrame* childFrame = frame->GetFirstPrincipalChild();
+    for (nsIFrame* childFrame = frame->PrincipalChildList().FirstChild();
          childFrame;
          childFrame = childFrame->GetNextSibling()) {
       // See if the child is a button control.
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index f3ca091fb0c..dde6b89dc43 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -1111,7 +1111,7 @@ nsGenericHTMLElement::GetFormControlFrame(bool aFlushFrames)
 
     // If we have generated content, the primary frame will be a
     // wrapper frame..  out real frame will be in its child list.
-    for (frame = frame->GetFirstPrincipalChild();
+    for (frame = frame->PrincipalChildList().FirstChild();
          frame;
          frame = frame->GetNextSibling()) {
       form_frame = do_QueryFrame(frame);
diff --git a/dom/html/nsTextEditorState.cpp b/dom/html/nsTextEditorState.cpp
index 06c88fac56a..11077c5149c 100644
--- a/dom/html/nsTextEditorState.cpp
+++ b/dom/html/nsTextEditorState.cpp
@@ -2168,7 +2168,7 @@ nsTextEditorState::InitializeKeyboardEventListeners()
                                     TrustedEventsAtSystemGroupBubble());
   }
 
-  mSelCon->SetScrollableFrame(do_QueryFrame(mBoundFrame->GetFirstPrincipalChild()));
+  mSelCon->SetScrollableFrame(do_QueryFrame(mBoundFrame->PrincipalChildList().FirstChild()));
 }
 
 void
diff --git a/layout/base/RestyleManager.cpp b/layout/base/RestyleManager.cpp
index be559cd075a..947048132da 100644
--- a/layout/base/RestyleManager.cpp
+++ b/layout/base/RestyleManager.cpp
@@ -223,13 +223,13 @@ GetFrameForChildrenOnlyTransformHint(nsIFrame *aFrame)
     // This happens if the root-<svg> is fixed positioned, in which case we
     // can't use aFrame->GetContent() to find the primary frame, since
     // GetContent() returns nullptr for ViewportFrame.
-    aFrame = aFrame->GetFirstPrincipalChild();
+    aFrame = aFrame->PrincipalChildList().FirstChild();
   }
   // For an nsHTMLScrollFrame, this will get the SVG frame that has the
   // children-only transforms:
   aFrame = aFrame->GetContent()->GetPrimaryFrame();
   if (aFrame->GetType() == nsGkAtoms::svgOuterSVGFrame) {
-    aFrame = aFrame->GetFirstPrincipalChild();
+    aFrame = aFrame->PrincipalChildList().FirstChild();
     MOZ_ASSERT(aFrame->GetType() == nsGkAtoms::svgOuterSVGAnonChildFrame,
                "Where is the nsSVGOuterSVGFrame's anon child??");
   }
@@ -327,7 +327,7 @@ DoApplyRenderingChangeToTree(nsIFrame* aFrame,
     if (aChange & nsChangeHint_ChildrenOnlyTransform) {
       needInvalidatingPaint = true;
       nsIFrame* childFrame =
-        GetFrameForChildrenOnlyTransformHint(aFrame)->GetFirstPrincipalChild();
+        GetFrameForChildrenOnlyTransformHint(aFrame)->PrincipalChildList().FirstChild();
       for ( ; childFrame; childFrame = childFrame->GetNextSibling()) {
         ActiveLayerTracker::NotifyRestyle(childFrame, eCSSProperty_transform);
       }
@@ -902,7 +902,7 @@ RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList)
         if (hint & nsChangeHint_ChildrenOnlyTransform) {
           // The overflow areas of the child frames need to be updated:
           nsIFrame* hintFrame = GetFrameForChildrenOnlyTransformHint(frame);
-          nsIFrame* childFrame = hintFrame->GetFirstPrincipalChild();
+          nsIFrame* childFrame = hintFrame->PrincipalChildList().FirstChild();
           NS_ASSERTION(!nsLayoutUtils::GetNextContinuationOrIBSplitSibling(frame),
                        "SVG frames should not have continuations "
                        "or ib-split siblings");
diff --git a/layout/base/nsBidiPresUtils.cpp b/layout/base/nsBidiPresUtils.cpp
index 789844db214..d296389090b 100644
--- a/layout/base/nsBidiPresUtils.cpp
+++ b/layout/base/nsBidiPresUtils.cpp
@@ -464,7 +464,7 @@ IsBidiSplittable(nsIFrame* aFrame)
 static bool
 IsBidiLeaf(nsIFrame* aFrame)
 {
-  nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
   return !kid || !aFrame->IsFrameOfType(nsIFrame::eBidiInlineContainer);
 }
 
@@ -670,7 +670,7 @@ nsBidiPresUtils::Resolve(nsBlockFrame* aBlockFrame)
     block->RemoveStateBits(NS_BLOCK_NEEDS_BIDI_RESOLUTION);
     nsBlockInFlowLineIterator lineIter(block, block->begin_lines());
     bpd.ResetForNewBlock();
-    TraverseFrames(aBlockFrame, &lineIter, block->GetFirstPrincipalChild(), &bpd);
+    TraverseFrames(aBlockFrame, &lineIter, block->PrincipalChildList().FirstChild(), &bpd);
     // XXX what about overflow lines?
   }
 
@@ -1184,7 +1184,7 @@ nsBidiPresUtils::TraverseFrames(nsBlockFrame*              aBlockFrame,
       }
     } else {
       // For a non-leaf frame, recurse into TraverseFrames
-      nsIFrame* kid = frame->GetFirstPrincipalChild();
+      nsIFrame* kid = frame->PrincipalChildList().FirstChild();
       MOZ_ASSERT(!frame->GetChildList(nsIFrame::kOverflowList).FirstChild(),
                  "should have drained the overflow list above");
       if (kid) {
@@ -1263,7 +1263,7 @@ nsBidiPresUtils::ReorderFrames(nsIFrame* aFirstFrameOnLine,
     // as the container size.
     containerSize = aFirstFrameOnLine->GetSize();
 
-    aFirstFrameOnLine = aFirstFrameOnLine->GetFirstPrincipalChild();
+    aFirstFrameOnLine = aFirstFrameOnLine->PrincipalChildList().FirstChild();
     if (!aFirstFrameOnLine) {
       return 0;
     }
@@ -1281,7 +1281,7 @@ nsBidiPresUtils::GetFirstLeaf(nsIFrame* aFrame)
 {
   nsIFrame* firstLeaf = aFrame;
   while (!IsBidiLeaf(firstLeaf)) {
-    nsIFrame* firstChild = firstLeaf->GetFirstPrincipalChild();
+    nsIFrame* firstChild = firstLeaf->PrincipalChildList().FirstChild();
     nsIFrame* realFrame = nsPlaceholderFrame::GetRealFrameFor(firstChild);
     firstLeaf = (realFrame->GetType() == nsGkAtoms::letterFrame) ?
                  realFrame : firstChild;
@@ -1307,7 +1307,7 @@ nsBidiPresUtils::GetFrameBaseLevel(nsIFrame* aFrame)
 {
   nsIFrame* firstLeaf = aFrame;
   while (!IsBidiLeaf(firstLeaf)) {
-    firstLeaf = firstLeaf->GetFirstPrincipalChild();
+    firstLeaf = firstLeaf->PrincipalChildList().FirstChild();
   }
   return NS_GET_BASE_LEVEL(firstLeaf);
 }
@@ -1651,7 +1651,7 @@ nsBidiPresUtils::InitContinuationStates(nsIFrame*              aFrame,
   if (!IsBidiLeaf(aFrame) || RubyUtils::IsRubyBox(aFrame->GetType())) {
     // Continue for child frames
     nsIFrame* frame;
-    for (frame = aFrame->GetFirstPrincipalChild();
+    for (frame = aFrame->PrincipalChildList().FirstChild();
          frame;
          frame = frame->GetNextSibling()) {
       InitContinuationStates(frame,
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index d678d6bda97..e4e3333098e 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -361,7 +361,7 @@ static inline nsContainerFrame*
 GetFieldSetBlockFrame(nsIFrame* aFieldsetFrame)
 {
   // Depends on the fieldset child frame order - see ConstructFieldSetFrame() below.
-  nsIFrame* firstChild = aFieldsetFrame->GetFirstPrincipalChild();
+  nsIFrame* firstChild = aFieldsetFrame->PrincipalChildList().FirstChild();
   nsIFrame* inner = firstChild && firstChild->GetNextSibling() ? firstChild->GetNextSibling() : firstChild;
   return inner ? inner->GetContentInsertionFrame() : nullptr;
 }
@@ -492,7 +492,7 @@ GetLastIBSplitSibling(nsIFrame* aFrame, bool aReturnEmptyTrailingInline)
   for (nsIFrame *frame = aFrame, *next; ; frame = next) {
     next = GetIBSplitSibling(frame);
     if (!next ||
-        (!aReturnEmptyTrailingInline && !next->GetFirstPrincipalChild() &&
+        (!aReturnEmptyTrailingInline && !next->PrincipalChildList().FirstChild() &&
          !GetIBSplitSibling(next))) {
       NS_ASSERTION(!next || !frame->IsInlineOutside(),
                    "Should have a block here!");
@@ -1867,9 +1867,9 @@ IsTablePseudo(nsIFrame* aFrame)
       aFrame->GetParent()->StyleContext()->GetPseudo() ==
         nsCSSAnonBoxes::tableCell) ||
      (pseudoType == nsCSSAnonBoxes::tableOuter &&
-      (aFrame->GetFirstPrincipalChild()->StyleContext()->GetPseudo() ==
+      (aFrame->PrincipalChildList().FirstChild()->StyleContext()->GetPseudo() ==
          nsCSSAnonBoxes::table ||
-       aFrame->GetFirstPrincipalChild()->StyleContext()->GetPseudo() ==
+       aFrame->PrincipalChildList().FirstChild()->StyleContext()->GetPseudo() ==
          nsCSSAnonBoxes::inlineTable)));
 }
 
@@ -2886,7 +2886,7 @@ nsCSSFrameConstructor::ConstructPageFrame(nsIPresShell*  aPresShell,
   // containing block for fixed elements which are repeated on every page.
   nsIFrame* prevPageContentFrame = nullptr;
   if (aPrevPageFrame) {
-    prevPageContentFrame = aPrevPageFrame->GetFirstPrincipalChild();
+    prevPageContentFrame = aPrevPageFrame->PrincipalChildList().FirstChild();
     NS_ASSERTION(prevPageContentFrame, "missing page content frame");
   }
   pageContentFrame->Init(nullptr, pageFrame, prevPageContentFrame);
@@ -2903,7 +2903,7 @@ nsCSSFrameConstructor::ConstructPageFrame(nsIPresShell*  aPresShell,
 
   nsIFrame* prevCanvasFrame = nullptr;
   if (prevPageContentFrame) {
-    prevCanvasFrame = prevPageContentFrame->GetFirstPrincipalChild();
+    prevCanvasFrame = prevPageContentFrame->PrincipalChildList().FirstChild();
     NS_ASSERTION(prevCanvasFrame, "missing canvas frame");
   }
   aCanvasFrame->Init(nullptr, pageContentFrame, prevCanvasFrame);
@@ -6182,7 +6182,7 @@ FindAppendPrevSibling(nsIFrame* aParentFrame, nsIFrame* aAfterFrame)
   if (aAfterFrame) {
     NS_ASSERTION(aAfterFrame->GetParent() == aParentFrame, "Wrong parent");
     NS_ASSERTION(aAfterFrame->GetPrevSibling() ||
-                 aParentFrame->GetFirstPrincipalChild() == aAfterFrame,
+                 aParentFrame->PrincipalChildList().FirstChild() == aAfterFrame,
                  ":after frame must be on the principal child list here");
     return aAfterFrame->GetPrevSibling();
   }
@@ -6203,7 +6203,7 @@ GetInsertNextSibling(nsIFrame* aParentFrame, nsIFrame* aPrevSibling)
     return aPrevSibling->GetNextSibling();
   }
 
-  return aParentFrame->GetFirstPrincipalChild();
+  return aParentFrame->PrincipalChildList().FirstChild();
 }
 
 /**
@@ -6220,7 +6220,7 @@ nsCSSFrameConstructor::AppendFramesToParent(nsFrameConstructorState&       aStat
 {
   NS_PRECONDITION(!IsFramePartOfIBSplit(aParentFrame) ||
                   !GetIBSplitSibling(aParentFrame) ||
-                  !GetIBSplitSibling(aParentFrame)->GetFirstPrincipalChild(),
+                  !GetIBSplitSibling(aParentFrame)->PrincipalChildList().FirstChild(),
                   "aParentFrame has a ib-split sibling with kids?");
   NS_PRECONDITION(!aPrevSibling || aPrevSibling->GetParent() == aParentFrame,
                   "Parent and prevsibling don't match");
@@ -6229,7 +6229,7 @@ nsCSSFrameConstructor::AppendFramesToParent(nsFrameConstructorState&       aStat
 
   NS_ASSERTION(nextSibling ||
                !aParentFrame->GetNextContinuation() ||
-               !aParentFrame->GetNextContinuation()->GetFirstPrincipalChild() ||
+               !aParentFrame->GetNextContinuation()->PrincipalChildList().FirstChild() ||
                aIsRecursiveCall,
                "aParentFrame has later continuations with kids?");
   NS_ASSERTION(nextSibling ||
@@ -7720,7 +7720,7 @@ nsCSSFrameConstructor::ContentRangeInserted(nsIContent*            aContainer,
   if (!prevSibling) {
     // We're inserting the new frames as the first child. See if the
     // parent has a :before pseudo-element
-    nsIFrame* firstChild = insertion.mParentFrame->GetFirstPrincipalChild();
+    nsIFrame* firstChild = insertion.mParentFrame->PrincipalChildList().FirstChild();
 
     if (firstChild &&
         nsLayoutUtils::IsGeneratedContentFor(container, firstChild,
@@ -8064,7 +8064,7 @@ nsCSSFrameConstructor::ContentRemoved(nsIContent*  aContainer,
   if (!aContainer) {
     nsIFrame* viewport = GetRootFrame();
     if (viewport) {
-      nsIFrame* firstChild = viewport->GetFirstPrincipalChild();
+      nsIFrame* firstChild = viewport->PrincipalChildList().FirstChild();
       if (firstChild && firstChild->GetContent() == aChild) {
         isRoot = true;
         childFrame = firstChild;
@@ -8146,7 +8146,7 @@ nsCSSFrameConstructor::ContentRemoved(nsIContent*  aContainer,
     if (grandparentFrame && grandparentFrame->IsBoxFrame() &&
         (grandparentFrame->GetStateBits() & NS_STATE_BOX_WRAPS_KIDS_IN_BLOCK) &&
         // check if this frame is the only one needing wrapping
-        aChild == AnyKidsNeedBlockParent(parentFrame->GetFirstPrincipalChild()) &&
+        aChild == AnyKidsNeedBlockParent(parentFrame->PrincipalChildList().FirstChild()) &&
         !AnyKidsNeedBlockParent(childFrame->GetNextSibling())) {
       *aDidReconstruct = true;
       LAYOUT_PHASE_TEMP_EXIT();
@@ -8538,7 +8538,7 @@ nsCSSFrameConstructor::CreateContinuingOuterTableFrame(nsIPresShell*     aPresSh
   // replicate the caption
   nsFrameItems  newChildFrames;
 
-  nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aFrame->PrincipalChildList().FirstChild();
   if (childFrame) {
     nsIFrame* continuingTableFrame =
       CreateContinuingFrame(aPresContext, childFrame, newFrame);
@@ -8566,7 +8566,7 @@ nsCSSFrameConstructor::CreateContinuingTableFrame(nsIPresShell*     aPresShell,
 
   // Replicate any header/footer frames
   nsFrameItems  childFrames;
-  nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aFrame->PrincipalChildList().FirstChild();
   for ( ; childFrame; childFrame = childFrame->GetNextSibling()) {
     // See if it's a header/footer, possibly wrapped in a scroll frame.
     nsTableRowGroupFrame* rowGroupFrame =
@@ -8689,7 +8689,7 @@ nsCSSFrameConstructor::CreateContinuingFrame(nsPresContext*    aPresContext,
 
     // Create a continuing frame for each table cell frame
     nsFrameItems  newChildList;
-    nsIFrame* cellFrame = aFrame->GetFirstPrincipalChild();
+    nsIFrame* cellFrame = aFrame->PrincipalChildList().FirstChild();
     while (cellFrame) {
       // See if it's a table cell frame
       if (IS_TABLE_CELL(cellFrame->GetType())) {
@@ -8718,7 +8718,7 @@ nsCSSFrameConstructor::CreateContinuingFrame(nsPresContext*    aPresContext,
     }
 
     // Create a continuing area frame
-    nsIFrame* blockFrame = aFrame->GetFirstPrincipalChild();
+    nsIFrame* blockFrame = aFrame->PrincipalChildList().FirstChild();
     nsIFrame* continuingBlockFrame =
       CreateContinuingFrame(aPresContext, blockFrame,
                             static_cast<nsContainerFrame*>(cellFrame));
@@ -8831,8 +8831,8 @@ nsCSSFrameConstructor::ReplicateFixedFrames(nsPageContentFrame* aParentFrame)
     return NS_OK;
   }
   nsContainerFrame* canvasFrame =
-    do_QueryFrame(aParentFrame->GetFirstPrincipalChild());
-  nsIFrame* prevCanvasFrame = prevPageContentFrame->GetFirstPrincipalChild();
+    do_QueryFrame(aParentFrame->PrincipalChildList().FirstChild());
+  nsIFrame* prevCanvasFrame = prevPageContentFrame->PrincipalChildList().FirstChild();
   if (!canvasFrame || !prevCanvasFrame) {
     // document's root element frame missing
     return NS_ERROR_UNEXPECTED;
@@ -8887,7 +8887,7 @@ nsCSSFrameConstructor::ReplicateFixedFrames(nsPageContentFrame* aParentFrame)
   // Add the placeholders to our primary child list.
   // XXXbz this is a little screwed up, since the fixed frames will have
   // broken auto-positioning. Oh, well.
-  NS_ASSERTION(!canvasFrame->GetFirstPrincipalChild(),
+  NS_ASSERTION(!canvasFrame->PrincipalChildList().FirstChild(),
                "leaking frames; doc root continuation must be empty");
   canvasFrame->SetInitialChildList(kPrincipalList, fixedPlaceholders);
   return NS_OK;
@@ -9034,7 +9034,7 @@ IsWhitespaceFrame(nsIFrame* aFrame)
 static nsIFrame*
 FindFirstNonWhitespaceChild(nsIFrame* aParentFrame)
 {
-  nsIFrame* f = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* f = aParentFrame->PrincipalChildList().FirstChild();
   while (f && IsWhitespaceFrame(f)) {
     f = f->GetNextSibling();
   }
@@ -9253,7 +9253,7 @@ nsCSSFrameConstructor::MaybeRecreateContainerForFrameRemoval(nsIFrame* aFrame,
 
   // If inFlowFrame is not the only in-flow child of |parent|, then removing
   // it will change nothing about the {ib} split.
-  if (inFlowFrame != parent->GetFirstPrincipalChild() ||
+  if (inFlowFrame != parent->PrincipalChildList().FirstChild() ||
       inFlowFrame->LastContinuation()->GetNextSibling()) {
     return false;
   }
@@ -10659,7 +10659,7 @@ nsCSSFrameConstructor::InsertFirstLineFrames(
   if (!aPrevSibling) {
     // Insertion will become the first frame. Two cases: we either
     // already have a first-line frame or we don't.
-    nsIFrame* firstBlockKid = aBlockFrame->GetFirstPrincipalChild();
+    nsIFrame* firstBlockKid = aBlockFrame->PrincipalChildList().FirstChild();
     if (firstBlockKid->GetType() == nsGkAtoms::lineFrame) {
       // We already have a first-line frame
       nsIFrame* lineFrame = firstBlockKid;
@@ -10759,7 +10759,7 @@ nsCSSFrameConstructor::InsertFirstLineFrames(
             if (!nextLineFrame) {
               break;
             }
-            nsIFrame* kids = nextLineFrame->GetFirstPrincipalChild();
+            nsIFrame* kids = nextLineFrame->PrincipalChildList().FirstChild();
           }
         }
         else {
@@ -11063,7 +11063,7 @@ nsCSSFrameConstructor::WrapFramesInFirstLetterFrame(
       }
     }
     else if (IsInlineFrame(frame) && frameType != nsGkAtoms::brFrame) {
-      nsIFrame* kids = frame->GetFirstPrincipalChild();
+      nsIFrame* kids = frame->PrincipalChildList().FirstChild();
       WrapFramesInFirstLetterFrame(aBlockFrame, aBlockContinuation,
                                    static_cast<nsContainerFrame*>(frame),
                                    kids, aModifiedParent, aTextFrame,
@@ -11118,7 +11118,7 @@ nsCSSFrameConstructor::RemoveFloatingFirstLetterFrames(
 
   // Take the text frame away from the letter frame (so it isn't
   // destroyed when we destroy the letter frame).
-  nsIFrame* textFrame = floatFrame->GetFirstPrincipalChild();
+  nsIFrame* textFrame = floatFrame->PrincipalChildList().FirstChild();
   if (!textFrame) {
     return NS_OK;
   }
@@ -11197,12 +11197,12 @@ nsCSSFrameConstructor::RemoveFirstLetterFrames(nsIPresShell* aPresShell,
                                                bool* aStopLooking)
 {
   nsIFrame* prevSibling = nullptr;
-  nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
 
   while (kid) {
     if (nsGkAtoms::letterFrame == kid->GetType()) {
       // Bingo. Found it. First steal away the text frame.
-      nsIFrame* textFrame = kid->GetFirstPrincipalChild();
+      nsIFrame* textFrame = kid->PrincipalChildList().FirstChild();
       if (!textFrame) {
         break;
       }
@@ -11309,7 +11309,7 @@ nsCSSFrameConstructor::RecoverLetterFrames(nsContainerFrame* aBlockFrame)
     // XXX shouldn't this bit be set already (bug 408493), assert instead?
     continuation->AddStateBits(NS_BLOCK_HAS_FIRST_LETTER_STYLE);
     WrapFramesInFirstLetterFrame(aBlockFrame, continuation, continuation,
-                                 continuation->GetFirstPrincipalChild(),
+                                 continuation->PrincipalChildList().FirstChild(),
                                  &parentFrame, &textFrame, &prevFrame,
                                  letterFrames, &stopLooking);
     if (stopLooking) {
@@ -12025,7 +12025,7 @@ nsCSSFrameConstructor::WipeContainingBlock(nsFrameConstructorState& aState,
               // Try to find one after all
               nsIFrame* parentNextCont = aFrame->GetNextContinuation();
               while (parentNextCont) {
-                nextSibling = parentNextCont->GetFirstPrincipalChild();
+                nextSibling = parentNextCont->PrincipalChildList().FirstChild();
                 if (nextSibling) {
                   break;
                 }
diff --git a/layout/base/nsCSSRendering.cpp b/layout/base/nsCSSRendering.cpp
index eb6d1726a16..027c0175e9f 100644
--- a/layout/base/nsCSSRendering.cpp
+++ b/layout/base/nsCSSRendering.cpp
@@ -3115,7 +3115,7 @@ nsCSSRendering::ComputeImageLayerPositioningArea(nsPresContext* aPresContext,
   }
 
   if (MOZ_UNLIKELY(frameType == nsGkAtoms::canvasFrame)) {
-    geometryFrame = aForFrame->GetFirstPrincipalChild();
+    geometryFrame = aForFrame->PrincipalChildList().FirstChild();
     // geometryFrame might be null if this canvas is a page created
     // as an overflow container (e.g. the in-flow content has already
     // finished and this page only displays the continuations of
diff --git a/layout/base/nsCaret.cpp b/layout/base/nsCaret.cpp
index 38a0026b1dd..27bf14f9b57 100644
--- a/layout/base/nsCaret.cpp
+++ b/layout/base/nsCaret.cpp
@@ -62,7 +62,7 @@ CheckForTrailingTextFrameRecursive(nsIFrame* aFrame, nsIFrame* aStopAtFrame)
   if (!aFrame->IsFrameOfType(nsIFrame::eLineParticipant))
     return nullptr;
 
-  for (nsIFrame* f = aFrame->GetFirstPrincipalChild(); f; f = f->GetNextSibling())
+  for (nsIFrame* f = aFrame->PrincipalChildList().FirstChild(); f; f = f->GetNextSibling())
   {
     nsIFrame* r = CheckForTrailingTextFrameRecursive(f, aStopAtFrame);
     if (r)
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 030f49b2676..30db3e06476 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -3795,7 +3795,7 @@ nsDocumentViewer::PrintPreviewNavigate(int16_t aType, int32_t aPageNum)
 
   // Now, locate the current page we are on and
   // and the page of the page number
-  nsIFrame* pageFrame = seqFrame->GetFirstPrincipalChild();
+  nsIFrame* pageFrame = seqFrame->PrincipalChildList().FirstChild();
   while (pageFrame != nullptr) {
     nsRect pageRect = pageFrame->GetRect();
     if (pageRect.Contains(pageRect.x, pt.y)) {
diff --git a/layout/base/nsFrameTraversal.cpp b/layout/base/nsFrameTraversal.cpp
index 51a30fdadd5..6ab455395eb 100644
--- a/layout/base/nsFrameTraversal.cpp
+++ b/layout/base/nsFrameTraversal.cpp
@@ -466,7 +466,7 @@ nsFrameIterator::GetPrevSibling(nsIFrame* aFrame)
 
 nsIFrame*
 nsFrameIterator::GetFirstChildInner(nsIFrame* aFrame) {
-  return aFrame->GetFirstPrincipalChild();
+  return aFrame->PrincipalChildList().FirstChild();
 }
 
 nsIFrame*
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 1a1bbd5bc16..8bd5278839c 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -1254,7 +1254,7 @@ nsLayoutUtils::LastContinuationWithChild(nsContainerFrame* aFrame)
 {
   NS_PRECONDITION(aFrame, "NULL frame pointer");
   nsIFrame* f = aFrame->LastContinuation();
-  while (!f->GetFirstPrincipalChild() && f->GetPrevContinuation()) {
+  while (!f->PrincipalChildList().FirstChild() && f->GetPrevContinuation()) {
     f = f->GetPrevContinuation();
   }
   return static_cast<nsContainerFrame*>(f);
@@ -1382,7 +1382,7 @@ nsLayoutUtils::GetBeforeFrameForContent(nsIFrame* aFrame,
   // If the first child frame is a pseudo-frame, then try that.
   // Note that the frame we create for the generated content is also a
   // pseudo-frame and so don't drill down in that case.
-  nsIFrame* childFrame = genConParentFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = genConParentFrame->PrincipalChildList().FirstChild();
   if (childFrame &&
       childFrame->IsPseudoFrame(aContent) &&
       !childFrame->IsGeneratedContentFrame()) {
@@ -1466,7 +1466,7 @@ nsIFrame*
 nsLayoutUtils::GetStyleFrame(nsIFrame* aFrame)
 {
   if (aFrame->GetType() == nsGkAtoms::tableOuterFrame) {
-    nsIFrame* inner = aFrame->GetFirstPrincipalChild();
+    nsIFrame* inner = aFrame->PrincipalChildList().FirstChild();
     NS_ASSERTION(inner, "Outer table must have an inner");
     return inner;
   }
@@ -3597,7 +3597,7 @@ AddBoxesForFrame(nsIFrame* aFrame,
   nsIAtom* pseudoType = aFrame->StyleContext()->GetPseudo();
 
   if (pseudoType == nsCSSAnonBoxes::tableOuter) {
-    AddBoxesForFrame(aFrame->GetFirstPrincipalChild(), aCallback);
+    AddBoxesForFrame(aFrame->PrincipalChildList().FirstChild(), aCallback);
     nsIFrame* kid = aFrame->GetChildList(nsIFrame::kCaptionList).FirstChild();
     if (kid) {
       AddBoxesForFrame(kid, aCallback);
@@ -3630,7 +3630,7 @@ nsLayoutUtils::GetFirstNonAnonymousFrame(nsIFrame* aFrame)
     nsIAtom* pseudoType = aFrame->StyleContext()->GetPseudo();
 
     if (pseudoType == nsCSSAnonBoxes::tableOuter) {
-      nsIFrame* f = GetFirstNonAnonymousFrame(aFrame->GetFirstPrincipalChild());
+      nsIFrame* f = GetFirstNonAnonymousFrame(aFrame->PrincipalChildList().FirstChild());
       if (f) {
         return f;
       }
@@ -5859,7 +5859,7 @@ nsLayoutUtils::GetFirstLinePosition(WritingMode aWM,
 
     if (fType == nsGkAtoms::fieldSetFrame) {
       LinePosition kidPosition;
-      nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+      nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
       // kid might be a legend frame here, but that's ok.
       if (GetFirstLinePosition(aWM, kid, &kidPosition)) {
         *aResult = kidPosition +
@@ -6758,7 +6758,7 @@ nsLayoutUtils::GetFrameTransparency(nsIFrame* aBackgroundFrame,
   // doing otherwise breaks window display effects on some platforms,
   // specifically Vista. (bug 450322)
   if (aBackgroundFrame->GetType() == nsGkAtoms::viewportFrame &&
-      !aBackgroundFrame->GetFirstPrincipalChild()) {
+      !aBackgroundFrame->PrincipalChildList().FirstChild()) {
     return eTransparencyOpaque;
   }
 
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index e46b096740b..223b9fca6a9 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -2276,7 +2276,7 @@ nsIPresShell::GetRootScrollFrame() const
   // Ensure root frame is a viewport frame
   if (!rootFrame || nsGkAtoms::viewportFrame != rootFrame->GetType())
     return nullptr;
-  nsIFrame* theFrame = rootFrame->GetFirstPrincipalChild();
+  nsIFrame* theFrame = rootFrame->PrincipalChildList().FirstChild();
   if (!theFrame || nsGkAtoms::scrollFrame != theFrame->GetType())
     return nullptr;
   return theFrame;
@@ -9591,7 +9591,7 @@ FindTopFrame(nsIFrame* aRoot)
     }
 
     // Try one of the children
-    nsIFrame* kid = aRoot->GetFirstPrincipalChild();
+    nsIFrame* kid = aRoot->PrincipalChildList().FirstChild();
     while (nullptr != kid) {
       nsIFrame* result = FindTopFrame(kid);
       if (nullptr != result) {
@@ -10177,7 +10177,7 @@ static void RecurseIndiTotals(nsPresContext* aPresContext,
     free(name);
   }
 
-  nsIFrame* child = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* child = aParentFrame->PrincipalChildList().FirstChild();
   while (child) {
     RecurseIndiTotals(aPresContext, aHT, child, aLevel+1);
     child = child->GetNextSibling();
diff --git a/layout/forms/nsHTMLButtonControlFrame.h b/layout/forms/nsHTMLButtonControlFrame.h
index 61bbe5e02eb..e5784202e08 100644
--- a/layout/forms/nsHTMLButtonControlFrame.h
+++ b/layout/forms/nsHTMLButtonControlFrame.h
@@ -81,7 +81,7 @@ public:
 
   // Inserted child content gets its frames parented by our child block
   virtual nsContainerFrame* GetContentInsertionFrame() override {
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
   virtual bool IsFrameOfType(uint32_t aFlags) const override
diff --git a/layout/forms/nsListControlFrame.cpp b/layout/forms/nsListControlFrame.cpp
index fafa31c535c..8d9c6258bab 100644
--- a/layout/forms/nsListControlFrame.cpp
+++ b/layout/forms/nsListControlFrame.cpp
@@ -265,7 +265,7 @@ static nscoord
 GetMaxOptionBSize(nsIFrame* aContainer, WritingMode aWM)
 {
   nscoord result = 0;
-  for (nsIFrame* option = aContainer->GetFirstPrincipalChild();
+  for (nsIFrame* option = aContainer->PrincipalChildList().FirstChild();
        option; option = option->GetNextSibling()) {
     nscoord optionBSize;
     if (nsCOMPtr<nsIDOMHTMLOptGroupElement>
diff --git a/layout/forms/nsTextControlFrame.cpp b/layout/forms/nsTextControlFrame.cpp
index fdbf641bcd3..36e8e66e7a9 100644
--- a/layout/forms/nsTextControlFrame.cpp
+++ b/layout/forms/nsTextControlFrame.cpp
@@ -210,7 +210,7 @@ nsTextControlFrame::CalcIntrinsicSize(nsRenderingContext* aRenderingContext,
 
   // Add in the size of the scrollbars for textarea
   if (IsTextArea()) {
-    nsIFrame* first = GetFirstPrincipalChild();
+    nsIFrame* first = PrincipalChildList().FirstChild();
 
     nsIScrollableFrame *scrollableFrame = do_QueryFrame(first);
     NS_ASSERTION(scrollableFrame, "Child must be scrollable");
@@ -1231,7 +1231,7 @@ nsTextControlFrame::SetInitialChildList(ChildListID     aListID,
   // Mark the scroll frame as being a reflow root. This will allow
   // incremental reflows to be initiated at the scroll frame, rather
   // than descending from the root frame of the frame hierarchy.
-  if (nsIFrame* first = GetFirstPrincipalChild()) {
+  if (nsIFrame* first = PrincipalChildList().FirstChild()) {
     first->AddStateBits(NS_FRAME_REFLOW_ROOT);
 
     nsCOMPtr<nsITextControlElement> txtCtrl = do_QueryInterface(GetContent());
diff --git a/layout/forms/nsTextControlFrame.h b/layout/forms/nsTextControlFrame.h
index 044fe14d6f2..0157d3b4a78 100644
--- a/layout/forms/nsTextControlFrame.h
+++ b/layout/forms/nsTextControlFrame.h
@@ -40,7 +40,7 @@ public:
   virtual void DestroyFrom(nsIFrame* aDestructRoot) override;
 
   virtual nsIScrollableFrame* GetScrollTargetFrame() override {
-    return do_QueryFrame(GetFirstPrincipalChild());
+    return do_QueryFrame(PrincipalChildList().FirstChild());
   }
 
   virtual nscoord GetMinISize(nsRenderingContext* aRenderingContext) override;
diff --git a/layout/generic/RubyUtils.cpp b/layout/generic/RubyUtils.cpp
index 3a9cdbc1ad8..f340663bc21 100644
--- a/layout/generic/RubyUtils.cpp
+++ b/layout/generic/RubyUtils.cpp
@@ -78,7 +78,7 @@ RubyColumn::Iterator::SkipUntilExistingFrame()
 
 RubySegmentEnumerator::RubySegmentEnumerator(nsRubyFrame* aRubyFrame)
 {
-  nsIFrame* frame = aRubyFrame->GetFirstPrincipalChild();
+  nsIFrame* frame = aRubyFrame->PrincipalChildList().FirstChild();
   MOZ_ASSERT(!frame ||
              frame->GetType() == nsGkAtoms::rubyBaseContainerFrame);
   mBaseContainer = static_cast<nsRubyBaseContainerFrame*>(frame);
@@ -103,7 +103,7 @@ RubyColumnEnumerator::RubyColumnEnumerator(
   const uint32_t rtcCount = aTextContainers.Length();
   mFrames.SetCapacity(rtcCount + 1);
 
-  nsIFrame* rbFrame = aBaseContainer->GetFirstPrincipalChild();
+  nsIFrame* rbFrame = aBaseContainer->PrincipalChildList().FirstChild();
   MOZ_ASSERT(!rbFrame || rbFrame->GetType() == nsGkAtoms::rubyBaseFrame);
   mFrames.AppendElement(static_cast<nsRubyContentFrame*>(rbFrame));
   for (uint32_t i = 0; i < rtcCount; i++) {
@@ -111,7 +111,7 @@ RubyColumnEnumerator::RubyColumnEnumerator(
     // If the container is for span, leave a nullptr here.
     // Spans do not take part in pairing.
     nsIFrame* rtFrame = !container->IsSpanContainer() ?
-      container->GetFirstPrincipalChild() : nullptr;
+      container->PrincipalChildList().FirstChild() : nullptr;
     MOZ_ASSERT(!rtFrame || rtFrame->GetType() == nsGkAtoms::rubyTextFrame);
     mFrames.AppendElement(static_cast<nsRubyContentFrame*>(rtFrame));
   }
diff --git a/layout/generic/TextOverflow.cpp b/layout/generic/TextOverflow.cpp
index afcfa915645..bee2350b1ae 100644
--- a/layout/generic/TextOverflow.cpp
+++ b/layout/generic/TextOverflow.cpp
@@ -380,7 +380,7 @@ TextOverflow::ExamineFrameSubtree(nsIFrame*       aFrame,
     return;
   }
 
-  nsIFrame* child = aFrame->GetFirstPrincipalChild();
+  nsIFrame* child = aFrame->PrincipalChildList().FirstChild();
   while (child) {
     ExamineFrameSubtree(child, aContentArea, aInsideMarkersArea,
                         aFramesToHide, aAlignmentEdges,
diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index bfb2a7fcbd0..02626e1d963 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -2368,7 +2368,7 @@ nsBlockFrame::ReflowDirtyLines(nsBlockReflowState& aState)
           aState.mPresContext->HasPendingInterrupt()) {
         // Need to make sure to pull overflows from any prev-in-flows
         for (nsIFrame* inlineKid = line->mFirstChild; inlineKid;
-             inlineKid = inlineKid->GetFirstPrincipalChild()) {
+             inlineKid = inlineKid->PrincipalChildList().FirstChild()) {
           inlineKid->PullOverflowsFromPrevInFlow();
         }
       }
@@ -7200,7 +7200,7 @@ nsBlockFrame::DoCollectFloats(nsIFrame* aFrame, nsFrameList& aList,
         // XXXmats nsInlineFrame's lazy reparenting depends on NOT doing that.
       }
 
-      DoCollectFloats(aFrame->GetFirstPrincipalChild(), aList, true);
+      DoCollectFloats(aFrame->PrincipalChildList().FirstChild(), aList, true);
       DoCollectFloats(aFrame->GetChildList(kOverflowList).FirstChild(), aList, true);
     }
     if (!aCollectSiblings)
diff --git a/layout/generic/nsBlockReflowContext.cpp b/layout/generic/nsBlockReflowContext.cpp
index 44aa2f02d16..6a046f26873 100644
--- a/layout/generic/nsBlockReflowContext.cpp
+++ b/layout/generic/nsBlockReflowContext.cpp
@@ -41,7 +41,7 @@ static nsIFrame* DescendIntoBlockLevelFrame(nsIFrame* aFrame)
   nsIAtom* type = aFrame->GetType();
   if (type == nsGkAtoms::columnSetFrame) {
     static_cast<nsColumnSetFrame*>(aFrame)->DrainOverflowColumns();
-    nsIFrame* child = aFrame->GetFirstPrincipalChild();
+    nsIFrame* child = aFrame->PrincipalChildList().FirstChild();
     if (child) {
       return DescendIntoBlockLevelFrame(child);
     }
diff --git a/layout/generic/nsCanvasFrame.cpp b/layout/generic/nsCanvasFrame.cpp
index cba64823e16..0989a5bbe5d 100644
--- a/layout/generic/nsCanvasFrame.cpp
+++ b/layout/generic/nsCanvasFrame.cpp
@@ -435,7 +435,7 @@ nsCanvasFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   }
 
   nsIFrame* kid;
-  for (kid = GetFirstPrincipalChild(); kid; kid = kid->GetNextSibling()) {
+  for (kid = PrincipalChildList().FirstChild(); kid; kid = kid->GetNextSibling()) {
     // Put our child into its own pseudo-stack.
     BuildDisplayListForChild(aBuilder, kid, aDirtyRect, aLists);
   }
diff --git a/layout/generic/nsColumnSetFrame.h b/layout/generic/nsColumnSetFrame.h
index 49860624d54..fefb298de56 100644
--- a/layout/generic/nsColumnSetFrame.h
+++ b/layout/generic/nsColumnSetFrame.h
@@ -45,7 +45,7 @@ public:
   virtual nscoord GetAvailableContentBSize(const nsHTMLReflowState& aReflowState);
 
   virtual nsContainerFrame* GetContentInsertionFrame() override {
-    nsIFrame* frame = GetFirstPrincipalChild();
+    nsIFrame* frame = PrincipalChildList().FirstChild();
 
     // if no children return nullptr
     if (!frame)
diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index c956a895671..af5d5b9ee1f 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -917,7 +917,7 @@ GetFirstNonAnonBoxDescendant(nsIFrame* aFrame)
     }
 
     // USUAL CASE: Descend to the first child in principal list.
-    aFrame = aFrame->GetFirstPrincipalChild();
+    aFrame = aFrame->PrincipalChildList().FirstChild();
   }
   return aFrame;
 }
@@ -2023,7 +2023,7 @@ nsFlexContainerFrame::SanityCheckAnonymousFlexItems() const
                  "two anon flex items in a row (shouldn't happen, unless our "
                  "children have been reordered with the 'order' property)");
 
-      nsIFrame* firstWrappedChild = child->GetFirstPrincipalChild();
+      nsIFrame* firstWrappedChild = child->PrincipalChildList().FirstChild();
       MOZ_ASSERT(firstWrappedChild,
                  "anonymous flex item is empty (shouldn't happen)");
       prevChildWasAnonFlexItem = true;
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index f64db90b82e..7f3bed7cf05 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -789,7 +789,7 @@ nsFrame::DidSetStyleContext(nsStyleContext* aOldStyleContext)
   if (IsSVGText()) {
     SVGTextFrame* svgTextFrame = static_cast<SVGTextFrame*>(
         nsLayoutUtils::GetClosestFrameOfType(this, nsGkAtoms::svgTextFrame));
-    nsIFrame* anonBlock = svgTextFrame->GetFirstPrincipalChild();
+    nsIFrame* anonBlock = svgTextFrame->PrincipalChildList().FirstChild();
     // Just as in SVGTextFrame::DidSetStyleContext, we need to ensure that
     // any non-display SVGTextFrames get reflowed when a child text frame
     // gets new style.
@@ -3675,7 +3675,7 @@ static FrameTarget DrillDownToSelectionFrame(nsIFrame* aFrame,
                                              bool aEndFrame, uint32_t aFlags) {
   if (SelectionDescendToKids(aFrame)) {
     nsIFrame* result = nullptr;
-    nsIFrame *frame = aFrame->GetFirstPrincipalChild();
+    nsIFrame *frame = aFrame->PrincipalChildList().FirstChild();
     if (!aEndFrame) {
       while (frame && (!SelfIsSelectable(frame, aFlags) ||
                         frame->IsEmpty()))
@@ -3858,7 +3858,7 @@ static FrameTarget GetSelectionClosestFrame(nsIFrame* aFrame, nsPoint aPoint,
       return target;
   }
 
-  nsIFrame *kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame *kid = aFrame->PrincipalChildList().FirstChild();
 
   if (kid) {
     // Go through all the child frames to find the closest one
@@ -6637,7 +6637,7 @@ FindBlockFrameOrBR(nsIFrame* aFrame, nsDirection aDirection)
       child = child->GetPrevSibling();
     }
   } else { // eDirNext
-    nsIFrame* child = aFrame->GetFirstPrincipalChild();
+    nsIFrame* child = aFrame->PrincipalChildList().FirstChild();
     while(child && !result.mContent) {
       result = FindBlockFrameOrBR(child, aDirection);
       child = child->GetNextSibling();
@@ -6950,7 +6950,7 @@ nsIFrame::PeekOffset(nsPeekOffsetStruct* aPos)
             if (aPos->mResultFrame->GetType() == nsGkAtoms::tableOuterFrame ||
                 aPos->mResultFrame->GetType() == nsGkAtoms::tableCellFrame)
             {
-              nsIFrame *frame = aPos->mResultFrame->GetFirstPrincipalChild();
+              nsIFrame *frame = aPos->mResultFrame->PrincipalChildList().FirstChild();
               //got the table frame now
               while(frame) //ok time to drill down to find iterator
               {
@@ -6963,7 +6963,7 @@ nsIFrame::PeekOffset(nsPeekOffsetStruct* aPos)
                   break; //while(frame)
                 }
                 result = NS_ERROR_FAILURE;
-                frame = frame->GetFirstPrincipalChild();
+                frame = frame->PrincipalChildList().FirstChild();
               }
             }
 
@@ -7627,7 +7627,7 @@ ComputeAndIncludeOutlineArea(nsIFrame* aFrame, nsOverflowAreas& aOverflowAreas,
         pseudoType != nsCSSAnonBoxes::mozAnonymousPositionedBlock)
       break;
     // If we're done, we really want it and all its later siblings.
-    frameForArea = frameForArea->GetFirstPrincipalChild();
+    frameForArea = frameForArea->PrincipalChildList().FirstChild();
     NS_ASSERTION(frameForArea, "anonymous block with no children?");
   } while (frameForArea);
 
@@ -8065,7 +8065,7 @@ GetCorrectedParent(const nsIFrame* aFrame)
   // For a table caption we want the _inner_ table frame (unless it's anonymous)
   // as the style parent.
   if (aFrame->IsTableCaption()) {
-    nsIFrame* innerTable = parent->GetFirstPrincipalChild();
+    nsIFrame* innerTable = parent->PrincipalChildList().FirstChild();
     if (!innerTable->StyleContext()->GetPseudo()) {
       return innerTable;
     }
@@ -8076,7 +8076,7 @@ GetCorrectedParent(const nsIFrame* aFrame)
   // know its parent.  So get the pseudo of the inner in that case.
   nsIAtom* pseudo = aFrame->StyleContext()->GetPseudo();
   if (pseudo == nsCSSAnonBoxes::tableOuter) {
-    pseudo = aFrame->GetFirstPrincipalChild()->StyleContext()->GetPseudo();
+    pseudo = aFrame->PrincipalChildList().FirstChild()->StyleContext()->GetPseudo();
   }
   return nsFrame::CorrectStyleParentFrame(parent, pseudo);
 }
@@ -8214,7 +8214,7 @@ nsFrame::GetLastLeaf(nsPresContext* aPresContext, nsIFrame **aFrame)
   nsIFrame *child = *aFrame;
   //if we are a block frame then go for the last line of 'this'
   while (1){
-    child = child->GetFirstPrincipalChild();
+    child = child->PrincipalChildList().FirstChild();
     if (!child)
       return;//nothing to do
     nsIFrame* siblingFrame;
@@ -8236,7 +8236,7 @@ nsFrame::GetFirstLeaf(nsPresContext* aPresContext, nsIFrame **aFrame)
     return;
   nsIFrame *child = *aFrame;
   while (1){
-    child = child->GetFirstPrincipalChild();
+    child = child->PrincipalChildList().FirstChild();
     if (!child)
       return;//nothing to do
     *aFrame = child;
diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index e2f8a4e5a61..f96c6a31e96 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -602,7 +602,7 @@ nsHTMLScrollFrame::GuessVScrollbarNeeded(const ScrollReflowState& aState)
     return false;
 
   if (mHelper.mIsRoot) {
-    nsIFrame *f = mHelper.mScrolledFrame->GetFirstPrincipalChild();
+    nsIFrame *f = mHelper.mScrolledFrame->PrincipalChildList().FirstChild();
     if (f && f->GetType() == nsGkAtoms::svgOuterSVGFrame &&
         static_cast<nsSVGOuterSVGFrame*>(f)->VerticalScrollbarNotNeeded()) {
       // Common SVG case - avoid a bad guess.
@@ -3913,7 +3913,7 @@ ScrollFrameHelper::ReloadChildFrames()
   mScrollCornerBox = nullptr;
   mResizerBox = nullptr;
 
-  nsIFrame* frame = mOuter->GetFirstPrincipalChild();
+  nsIFrame* frame = mOuter->PrincipalChildList().FirstChild();
   while (frame) {
     nsIContent* content = frame->GetContent();
     if (content == mOuter->GetContent()) {
@@ -5337,7 +5337,7 @@ ScrollFrameHelper::GetScrolledRectInternal(const nsRect& aScrolledFrameOverflowA
   // direction set by the text content overrides the direction of the frame
   if (mScrolledFrame->StyleTextReset()->mUnicodeBidi &
       NS_STYLE_UNICODE_BIDI_PLAINTEXT) {
-    nsIFrame* childFrame = mScrolledFrame->GetFirstPrincipalChild();
+    nsIFrame* childFrame = mScrolledFrame->PrincipalChildList().FirstChild();
     if (childFrame) {
       frameDir =
         (nsBidiPresUtils::ParagraphDirection(childFrame) == NSBIDI_LTR)
diff --git a/layout/generic/nsGridContainerFrame.cpp b/layout/generic/nsGridContainerFrame.cpp
index b5308b6addf..1c9b3819a5e 100644
--- a/layout/generic/nsGridContainerFrame.cpp
+++ b/layout/generic/nsGridContainerFrame.cpp
@@ -3585,7 +3585,7 @@ nsGridContainerFrame::SanityCheckAnonymousGridItems() const
                    "children have been reordered with the 'order' property)");
 */
         MOZ_ASSERT(!prevChildWasAnonGridItem, "two anon grid items in a row");
-        nsIFrame* firstWrappedChild = child->GetFirstPrincipalChild();
+        nsIFrame* firstWrappedChild = child->PrincipalChildList().FirstChild();
         MOZ_ASSERT(firstWrappedChild,
                    "anonymous grid item is empty (shouldn't happen)");
         prevChildWasAnonGridItem = true;
diff --git a/layout/generic/nsHTMLCanvasFrame.h b/layout/generic/nsHTMLCanvasFrame.h
index e3bbf731c37..aada81e0410 100644
--- a/layout/generic/nsHTMLCanvasFrame.h
+++ b/layout/generic/nsHTMLCanvasFrame.h
@@ -95,7 +95,7 @@ public:
 
   // Inserted child content gets its frames parented by our child block
   virtual nsContainerFrame* GetContentInsertionFrame() override {
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
 protected:
diff --git a/layout/generic/nsHTMLReflowState.cpp b/layout/generic/nsHTMLReflowState.cpp
index 85c430d01bb..d297a41645d 100644
--- a/layout/generic/nsHTMLReflowState.cpp
+++ b/layout/generic/nsHTMLReflowState.cpp
@@ -570,7 +570,7 @@ nsHTMLReflowState::InitResizeFlags(nsPresContext* aPresContext, nsIAtom* aFrameT
       if (frame->GetType() == nsGkAtoms::svgForeignObjectFrame) {
         // Foreign object frames use dirty bits in a special way.
         frame->AddStateBits(NS_FRAME_HAS_DIRTY_CHILDREN);
-        nsIFrame *kid = frame->GetFirstPrincipalChild();
+        nsIFrame *kid = frame->PrincipalChildList().FirstChild();
         if (kid) {
           kid->AddStateBits(NS_FRAME_IS_DIRTY);
         }
diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index 1f7ba243fc6..c4b5e5fbc1e 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -1088,10 +1088,6 @@ public:
    */
   void GetCrossDocChildLists(nsTArray<ChildList>* aLists);
 
-  nsIFrame* GetFirstPrincipalChild() const {
-    return GetChildList(kPrincipalList).FirstChild();
-  }
-
   // The individual concrete child lists.
   static const ChildListID kPrincipalList = mozilla::layout::kPrincipalList;
   static const ChildListID kAbsoluteList = mozilla::layout::kAbsoluteList;
diff --git a/layout/generic/nsInlineFrame.cpp b/layout/generic/nsInlineFrame.cpp
index dbafb16a387..e5925bd3aa3 100644
--- a/layout/generic/nsInlineFrame.cpp
+++ b/layout/generic/nsInlineFrame.cpp
@@ -652,7 +652,7 @@ nsInlineFrame::ReflowFrames(nsPresContext* aPresContext,
       // so nsFirstLetterFrame::Reflow can destroy them safely (bug 401042).
       nsIFrame* realFrame = nsPlaceholderFrame::GetRealFrameFor(frame);
       if (realFrame->GetType() == nsGkAtoms::letterFrame) {
-        nsIFrame* child = realFrame->GetFirstPrincipalChild();
+        nsIFrame* child = realFrame->PrincipalChildList().FirstChild();
         if (child) {
           NS_ASSERTION(child->GetType() == nsGkAtoms::textFrame,
                        "unexpected frame type");
diff --git a/layout/generic/nsPageFrame.cpp b/layout/generic/nsPageFrame.cpp
index 1a8c5721d3c..cba5dd65562 100644
--- a/layout/generic/nsPageFrame.cpp
+++ b/layout/generic/nsPageFrame.cpp
@@ -465,7 +465,7 @@ GetNextPage(nsIFrame* aPageContentFrame)
     return nullptr;
   NS_ASSERTION(nextPageFrame->GetType() == nsGkAtoms::pageFrame,
                "pageFrame's sibling is not a page frame...");
-  nsIFrame* f = nextPageFrame->GetFirstPrincipalChild();
+  nsIFrame* f = nextPageFrame->PrincipalChildList().FirstChild();
   NS_ASSERTION(f, "pageFrame has no page content frame!");
   NS_ASSERTION(f->GetType() == nsGkAtoms::pageContentFrame,
                "pageFrame's child is not page content!");
diff --git a/layout/generic/nsPluginFrame.cpp b/layout/generic/nsPluginFrame.cpp
index 89076f2e26d..fa4ddfa2f2d 100644
--- a/layout/generic/nsPluginFrame.cpp
+++ b/layout/generic/nsPluginFrame.cpp
@@ -1771,7 +1771,7 @@ nsPluginFrame::SetIsDocumentActive(bool aIsActive)
 nsIObjectFrame *
 nsPluginFrame::GetNextObjectFrame(nsPresContext* aPresContext, nsIFrame* aRoot)
 {
-  nsIFrame* child = aRoot->GetFirstPrincipalChild();
+  nsIFrame* child = aRoot->PrincipalChildList().FirstChild();
 
   while (child) {
     nsIObjectFrame* outFrame = do_QueryFrame(child);
diff --git a/layout/generic/nsRubyBaseContainerFrame.cpp b/layout/generic/nsRubyBaseContainerFrame.cpp
index 047e0cd27ae..364324babed 100644
--- a/layout/generic/nsRubyBaseContainerFrame.cpp
+++ b/layout/generic/nsRubyBaseContainerFrame.cpp
@@ -68,7 +68,7 @@ LineBreakBefore(nsIFrame* aFrame,
                 const nsLineList::iterator* aLine)
 {
   for (nsIFrame* child = aFrame; child;
-       child = child->GetFirstPrincipalChild()) {
+       child = child->PrincipalChildList().FirstChild()) {
     if (!child->CanContinueTextRun()) {
       // It is not an inline element. We can break before it.
       return gfxBreakPriority::eNormalBreak;
@@ -241,7 +241,7 @@ nsRubyBaseContainerFrame::AddInlinePrefISize(
   }
   for (uint32_t i = 0, iend = textContainers.Length(); i < iend; i++) {
     if (textContainers[i]->IsSpanContainer()) {
-      nsIFrame* frame = textContainers[i]->GetFirstPrincipalChild();
+      nsIFrame* frame = textContainers[i]->PrincipalChildList().FirstChild();
       nsIFrame::InlinePrefISizeData data;
       frame->AddInlinePrefISize(aRenderingContext, &data);
       MOZ_ASSERT(data.prevLines == 0, "Shouldn't have prev lines");
@@ -818,7 +818,7 @@ nsRubyBaseContainerFrame::ReflowSpans(const ReflowState& aReflowState)
       continue;
     }
 
-    nsIFrame* rtFrame = container->GetFirstPrincipalChild();
+    nsIFrame* rtFrame = container->PrincipalChildList().FirstChild();
     nsReflowStatus reflowStatus;
     bool pushedFrame;
     nsLineLayout* lineLayout = aReflowState.mTextReflowStates[i]->mLineLayout;
diff --git a/layout/generic/nsSimplePageSequenceFrame.cpp b/layout/generic/nsSimplePageSequenceFrame.cpp
index 103f62de04f..6f3edd2fed1 100644
--- a/layout/generic/nsSimplePageSequenceFrame.cpp
+++ b/layout/generic/nsSimplePageSequenceFrame.cpp
@@ -520,7 +520,7 @@ GetPrintCanvasElementsInFrame(nsIFrame* aFrame, nsTArray<RefPtr<HTMLCanvasElemen
         }
       }
 
-      if (!child->GetFirstPrincipalChild()) {
+      if (!child->PrincipalChildList().FirstChild()) {
         nsSubDocumentFrame* subdocumentFrame = do_QueryFrame(child);
         if (subdocumentFrame) {
           // Descend into the subdocument
@@ -742,7 +742,7 @@ nsSimplePageSequenceFrame::PrintNextPage()
     height -= mMargin.top + mMargin.bottom;
     width  -= mMargin.left + mMargin.right;
     nscoord selectionY = height;
-    nsIFrame* conFrame = currentPage->GetFirstPrincipalChild();
+    nsIFrame* conFrame = currentPage->PrincipalChildList().FirstChild();
     if (mSelectionHeight >= 0) {
       conFrame->SetPosition(conFrame->GetPosition() + nsPoint(0, -mYSelOffset));
       nsContainerFrame::PositionChildViews(conFrame);
@@ -837,7 +837,7 @@ nsSimplePageSequenceFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
     DisplayListClipState::AutoSaveRestore clipState(aBuilder);
     clipState.Clear();
 
-    nsIFrame* child = GetFirstPrincipalChild();
+    nsIFrame* child = PrincipalChildList().FirstChild();
     nsRect dirty = aDirtyRect;
     dirty.ScaleInverseRoundOut(PresContext()->GetPrintPreviewScale());
 
diff --git a/layout/generic/nsSubDocumentFrame.cpp b/layout/generic/nsSubDocumentFrame.cpp
index fc6ddf26330..23d49ca59f3 100644
--- a/layout/generic/nsSubDocumentFrame.cpp
+++ b/layout/generic/nsSubDocumentFrame.cpp
@@ -1270,7 +1270,7 @@ nsSubDocumentFrame::ObtainIntrinsicSizeFrame()
         if (scrollable) {
           nsIFrame* scrolled = scrollable->GetScrolledFrame();
           if (scrolled) {
-            subDocRoot = scrolled->GetFirstPrincipalChild();
+            subDocRoot = scrolled->PrincipalChildList().FirstChild();
           }
         }
       }
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index eda5181f2dc..8010ecac81c 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -1145,7 +1145,7 @@ CanTextCrossFrameBoundary(nsIFrame* aFrame, nsIAtom* aType)
     }
   } else {
     if (continuesTextRun) {
-      result.mFrameToScan = aFrame->GetFirstPrincipalChild();
+      result.mFrameToScan = aFrame->PrincipalChildList().FirstChild();
       result.mOverflowFrameToScan =
         aFrame->GetChildList(nsIFrame::kOverflowList).FirstChild();
       NS_WARN_IF_FALSE(!result.mOverflowFrameToScan,
diff --git a/layout/mathml/nsMathMLContainerFrame.cpp b/layout/mathml/nsMathMLContainerFrame.cpp
index a48312efd61..eeaa9cfe7a4 100644
--- a/layout/mathml/nsMathMLContainerFrame.cpp
+++ b/layout/mathml/nsMathMLContainerFrame.cpp
@@ -218,7 +218,7 @@ nsMathMLContainerFrame::GetPreferredStretchSize(DrawTarget*          aDrawTarget
     bool firstTime = true;
     nsBoundingMetrics bm, bmChild;
     nsIFrame* childFrame =
-      stretchAll ? GetFirstPrincipalChild() : mPresentationData.baseFrame;
+      stretchAll ? PrincipalChildList().FirstChild() : mPresentationData.baseFrame;
     while (childFrame) {
       // initializations in case this child happens not to be a MathML frame
       nsIMathMLFrame* mathMLFrame = do_QueryFrame(childFrame);
@@ -482,7 +482,7 @@ nsMathMLContainerFrame::FinalizeReflow(DrawTarget* aDrawTarget,
   // through Stretch() eventually.
   if (NS_MATHML_HAS_ERROR(mPresentationData.flags) || NS_FAILED(rv)) {
     GatherAndStoreOverflow(&aDesiredSize);
-    DidReflowChildren(GetFirstPrincipalChild());
+    DidReflowChildren(PrincipalChildList().FirstChild());
     return rv;
   }
 
@@ -531,7 +531,7 @@ nsMathMLContainerFrame::FinalizeReflow(DrawTarget* aDrawTarget,
       {
         // The Place() call above didn't request FinishReflowChild(),
         // so let's check that we eventually did through Stretch().
-        nsIFrame* childFrame = GetFirstPrincipalChild();
+        nsIFrame* childFrame = PrincipalChildList().FirstChild();
         for ( ; childFrame; childFrame = childFrame->GetNextSibling()) {
           NS_ASSERTION(!(childFrame->GetStateBits() & NS_FRAME_IN_REFLOW),
                        "DidReflow() was never called");
@@ -587,7 +587,7 @@ nsMathMLContainerFrame::PropagatePresentationDataFor(nsIFrame*       aFrame,
   }
   else {
     // propagate down the subtrees
-    nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
+    nsIFrame* childFrame = aFrame->PrincipalChildList().FirstChild();
     while (childFrame) {
       PropagatePresentationDataFor(childFrame,
         aFlagsValues, aFlagsToUpdate);
@@ -606,7 +606,7 @@ nsMathMLContainerFrame::PropagatePresentationDataFromChildAt(nsIFrame*       aPa
   if (!aParentFrame || !aFlagsToUpdate)
     return;
   int32_t index = 0;
-  nsIFrame* childFrame = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aParentFrame->PrincipalChildList().FirstChild();
   while (childFrame) {
     if ((index >= aFirstChildIndex) &&
         ((aLastChildIndex <= 0) || ((aLastChildIndex > 0) &&
@@ -666,7 +666,7 @@ nsMathMLContainerFrame::RebuildAutomaticDataForChildren(nsIFrame* aParentFrame)
   // the parent
   // 2. As we ascend the tree, transmit any specific change that we want
   // down the subtrees
-  nsIFrame* childFrame = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aParentFrame->PrincipalChildList().FirstChild();
   while (childFrame) {
     nsIMathMLFrame* childMathMLFrame = do_QueryFrame(childFrame);
     if (childMathMLFrame) {
@@ -1375,7 +1375,7 @@ GetInterFrameSpacingFor(int32_t         aScriptLevel,
                         nsIFrame*       aParentFrame,
                         nsIFrame*       aChildFrame)
 {
-  nsIFrame* childFrame = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aParentFrame->PrincipalChildList().FirstChild();
   if (!childFrame || aChildFrame == childFrame)
     return 0;
 
@@ -1463,7 +1463,7 @@ nsMathMLContainerFrame::DidReflowChildren(nsIFrame* aFirst, nsIFrame* aStop)
     NS_ASSERTION(frame, "aStop isn't a sibling");
     if (frame->GetStateBits() & NS_FRAME_IN_REFLOW) {
       // finish off principal descendants, too
-      nsIFrame* grandchild = frame->GetFirstPrincipalChild();
+      nsIFrame* grandchild = frame->PrincipalChildList().FirstChild();
       if (grandchild)
         DidReflowChildren(grandchild, nullptr);
 
@@ -1501,7 +1501,7 @@ nsMathMLContainerFrame::TransmitAutomaticDataForMrowLikeElement()
   bool embellishedOpFound = false;
   nsEmbellishData embellishData;
   
-  for (childFrame = GetFirstPrincipalChild();
+  for (childFrame = PrincipalChildList().FirstChild();
        childFrame;
        childFrame = childFrame->GetNextSibling()) {
     nsIMathMLFrame* mathMLFrame = do_QueryFrame(childFrame);
@@ -1555,7 +1555,7 @@ nsMathMLContainerFrame::PropagateFrameFlagFor(nsIFrame* aFrame,
     return;
 
   aFrame->AddStateBits(aFlags);
-  nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aFrame->PrincipalChildList().FirstChild();
   while (childFrame) {
     PropagateFrameFlagFor(childFrame, aFlags);
     childFrame = childFrame->GetNextSibling();
diff --git a/layout/mathml/nsMathMLTokenFrame.cpp b/layout/mathml/nsMathMLTokenFrame.cpp
index 73d58b28e9e..2c91fd0c065 100644
--- a/layout/mathml/nsMathMLTokenFrame.cpp
+++ b/layout/mathml/nsMathMLTokenFrame.cpp
@@ -64,9 +64,9 @@ nsMathMLTokenFrame::MarkTextFramesAsTokenMathML()
   // - to force them to trim their leading and trailing whitespaces.
   // - Indicate which frames are suitable for mathvariant
   // - flag single character <mi> frames for special italic treatment
-  for (nsIFrame* childFrame = GetFirstPrincipalChild(); childFrame;
+  for (nsIFrame* childFrame = PrincipalChildList().FirstChild(); childFrame;
        childFrame = childFrame->GetNextSibling()) {
-    for (nsIFrame* childFrame2 = childFrame->GetFirstPrincipalChild();
+    for (nsIFrame* childFrame2 = childFrame->PrincipalChildList().FirstChild();
          childFrame2; childFrame2 = childFrame2->GetNextSibling()) {
       if (childFrame2->GetType() == nsGkAtoms::textFrame) {
         childFrame2->AddStateBits(TEXT_IS_IN_TOKEN_MATHML);
@@ -132,7 +132,7 @@ nsMathMLTokenFrame::Reflow(nsPresContext*          aPresContext,
   aDesiredSize.SetBlockStartAscent(0);
   aDesiredSize.mBoundingMetrics = nsBoundingMetrics();
 
-  nsIFrame* childFrame = GetFirstPrincipalChild();
+  nsIFrame* childFrame = PrincipalChildList().FirstChild();
   while (childFrame) {
     // ask our children to compute their bounding metrics
     nsHTMLReflowMetrics childDesiredSize(aReflowState.GetWritingMode(),
@@ -168,7 +168,7 @@ nsMathMLTokenFrame::Place(DrawTarget*          aDrawTarget,
                           nsHTMLReflowMetrics& aDesiredSize)
 {
   mBoundingMetrics = nsBoundingMetrics();
-  for (nsIFrame* childFrame = GetFirstPrincipalChild(); childFrame;
+  for (nsIFrame* childFrame = PrincipalChildList().FirstChild(); childFrame;
        childFrame = childFrame->GetNextSibling()) {
     nsHTMLReflowMetrics childSize(aDesiredSize.GetWritingMode());
     GetReflowAndBoundingMetricsFor(childFrame, childSize,
@@ -192,7 +192,7 @@ nsMathMLTokenFrame::Place(DrawTarget*          aDrawTarget,
 
   if (aPlaceOrigin) {
     nscoord dy, dx = 0;
-    for (nsIFrame* childFrame = GetFirstPrincipalChild(); childFrame;
+    for (nsIFrame* childFrame = PrincipalChildList().FirstChild(); childFrame;
          childFrame = childFrame->GetNextSibling()) {
       nsHTMLReflowMetrics childSize(aDesiredSize.GetWritingMode());
       GetReflowAndBoundingMetricsFor(childFrame, childSize,
diff --git a/layout/mathml/nsMathMLmencloseFrame.cpp b/layout/mathml/nsMathMLmencloseFrame.cpp
index 149e9b5cfc7..9dc02df765f 100644
--- a/layout/mathml/nsMathMLmencloseFrame.cpp
+++ b/layout/mathml/nsMathMLmencloseFrame.cpp
@@ -327,7 +327,7 @@ nsMathMLmencloseFrame::PlaceInternal(DrawTarget*          aDrawTarget,
     nsMathMLContainerFrame::Place(aDrawTarget, false, baseSize);
 
   if (NS_MATHML_HAS_ERROR(mPresentationData.flags) || NS_FAILED(rv)) {
-      DidReflowChildren(GetFirstPrincipalChild());
+      DidReflowChildren(PrincipalChildList().FirstChild());
       return rv;
     }
 
diff --git a/layout/mathml/nsMathMLmfencedFrame.cpp b/layout/mathml/nsMathMLmfencedFrame.cpp
index 56d665f0a87..2d39e3f0fe6 100644
--- a/layout/mathml/nsMathMLmfencedFrame.cpp
+++ b/layout/mathml/nsMathMLmfencedFrame.cpp
@@ -245,7 +245,7 @@ nsMathMLmfencedFrame::Reflow(nsPresContext*          aPresContext,
   // refactored to use nsMathMLContainerFrame::Reflow() at some stage.
 
   nsReflowStatus childStatus;
-  nsIFrame* firstChild = GetFirstPrincipalChild();
+  nsIFrame* firstChild = PrincipalChildList().FirstChild();
   nsIFrame* childFrame = firstChild;
   nscoord ascent = 0, descent = 0;
   if (firstChild || mOpenChar || mCloseChar || mSeparatorsCount > 0) {
@@ -626,7 +626,7 @@ nsMathMLmfencedFrame::GetIntrinsicISizeMetrics(nsRenderingContext* aRenderingCon
   }
 
   int32_t i = 0;
-  nsIFrame* childFrame = GetFirstPrincipalChild();
+  nsIFrame* childFrame = PrincipalChildList().FirstChild();
   while (childFrame) {
     // XXX This includes margin while Reflow currently doesn't consider
     // margin, so we may end up with too much space, but, with stretchy
diff --git a/layout/mathml/nsMathMLmmultiscriptsFrame.cpp b/layout/mathml/nsMathMLmmultiscriptsFrame.cpp
index c1d9957a3c9..126f54ca305 100644
--- a/layout/mathml/nsMathMLmmultiscriptsFrame.cpp
+++ b/layout/mathml/nsMathMLmmultiscriptsFrame.cpp
@@ -173,7 +173,7 @@ nsMathMLmmultiscriptsFrame::PlaceMultiScript(nsPresContext*  aPresContext,
   // depend only on the current font
   ////////////////////////////////////////
 
-  nsIFrame* baseFrame = aFrame->GetFirstPrincipalChild();
+  nsIFrame* baseFrame = aFrame->PrincipalChildList().FirstChild();
 
   if (!baseFrame) {
     if (tag == nsGkAtoms::mmultiscripts_)
@@ -330,7 +330,7 @@ nsMathMLmmultiscriptsFrame::PlaceMultiScript(nsPresContext*  aPresContext,
   // Note that only msup starts with a superscript.
   bool isSubScript = (tag != nsGkAtoms::msup_);
 
-  nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aFrame->PrincipalChildList().FirstChild();
   while (childFrame) {
     if (childFrame->GetContent()->IsMathMLElement(nsGkAtoms::mprescripts_)) {
       if (tag != nsGkAtoms::mmultiscripts_) {
diff --git a/layout/mathml/nsMathMLmpaddedFrame.cpp b/layout/mathml/nsMathMLmpaddedFrame.cpp
index aefdabd8e4a..b24f93e741b 100644
--- a/layout/mathml/nsMathMLmpaddedFrame.cpp
+++ b/layout/mathml/nsMathMLmpaddedFrame.cpp
@@ -326,7 +326,7 @@ nsMathMLmpaddedFrame::Place(DrawTarget*          aDrawTarget,
   nsresult rv =
     nsMathMLContainerFrame::Place(aDrawTarget, false, aDesiredSize);
   if (NS_MATHML_HAS_ERROR(mPresentationData.flags) || NS_FAILED(rv)) {
-    DidReflowChildren(GetFirstPrincipalChild());
+    DidReflowChildren(PrincipalChildList().FirstChild());
     return rv;
   }
 
diff --git a/layout/mathml/nsMathMLmrowFrame.cpp b/layout/mathml/nsMathMLmrowFrame.cpp
index 5bdcff1706a..65b9e0fd5f0 100644
--- a/layout/mathml/nsMathMLmrowFrame.cpp
+++ b/layout/mathml/nsMathMLmrowFrame.cpp
@@ -43,7 +43,7 @@ nsMathMLmrowFrame::AttributeChanged(int32_t  aNameSpaceID,
   // notification to the real mtable
   if (mContent->IsMathMLElement(nsGkAtoms::mtable_)) {
     nsIFrame* frame = mFrames.FirstChild();
-    for ( ; frame; frame = frame->GetFirstPrincipalChild()) {
+    for ( ; frame; frame = frame->PrincipalChildList().FirstChild()) {
       // drill down to the real mtable
       if (frame->GetType() == nsGkAtoms::tableOuterFrame)
         return frame->AttributeChanged(aNameSpaceID, aAttribute, aModType);
diff --git a/layout/mathml/nsMathMLmtableFrame.cpp b/layout/mathml/nsMathMLmtableFrame.cpp
index 1df2a7188f9..912d0c9fd8f 100644
--- a/layout/mathml/nsMathMLmtableFrame.cpp
+++ b/layout/mathml/nsMathMLmtableFrame.cpp
@@ -577,11 +577,11 @@ MapAllAttributesIntoCSS(nsMathMLmtableFrame* aTableFrame)
   ParseSpacingAttributes(aTableFrame);
 
   // mtable is simple and only has one (pseudo) row-group
-  nsIFrame* rgFrame = aTableFrame->GetFirstPrincipalChild();
+  nsIFrame* rgFrame = aTableFrame->PrincipalChildList().FirstChild();
   if (!rgFrame || rgFrame->GetType() != nsGkAtoms::tableRowGroupFrame)
     return;
 
-  nsIFrame* rowFrame = rgFrame->GetFirstPrincipalChild();
+  nsIFrame* rowFrame = rgFrame->PrincipalChildList().FirstChild();
   for ( ; rowFrame; rowFrame = rowFrame->GetNextSibling()) {
     DEBUG_VERIFY_THAT_FRAME_IS(rowFrame, TABLE_ROW);
     if (rowFrame->GetType() == nsGkAtoms::tableRowFrame) {
@@ -590,7 +590,7 @@ MapAllAttributesIntoCSS(nsMathMLmtableFrame* aTableFrame)
       // Map row columnalign.
       ParseFrameAttribute(rowFrame, nsGkAtoms::columnalign_, true);
 
-      nsIFrame* cellFrame = rowFrame->GetFirstPrincipalChild();
+      nsIFrame* cellFrame = rowFrame->PrincipalChildList().FirstChild();
       for ( ; cellFrame; cellFrame = cellFrame->GetNextSibling()) {
         DEBUG_VERIFY_THAT_FRAME_IS(cellFrame, TABLE_CELL);
         if (IS_TABLE_CELL(cellFrame->GetType())) {
@@ -725,7 +725,7 @@ nsMathMLmtableOuterFrame::AttributeChanged(int32_t  aNameSpaceID,
   nsIFrame* tableFrame = mFrames.FirstChild();
   NS_ASSERTION(tableFrame && tableFrame->GetType() == nsGkAtoms::tableFrame,
                "should always have an inner table frame");
-  nsIFrame* rgFrame = tableFrame->GetFirstPrincipalChild();
+  nsIFrame* rgFrame = tableFrame->PrincipalChildList().FirstChild();
   if (!rgFrame || rgFrame->GetType() != nsGkAtoms::tableRowGroupFrame)
     return NS_OK;
 
@@ -798,7 +798,7 @@ nsMathMLmtableOuterFrame::GetRowFrameAt(int32_t aRowIndex)
     nsIFrame* tableFrame = mFrames.FirstChild();
     NS_ASSERTION(tableFrame && tableFrame->GetType() == nsGkAtoms::tableFrame,
                  "should always have an inner table frame");
-    nsIFrame* rgFrame = tableFrame->GetFirstPrincipalChild();
+    nsIFrame* rgFrame = tableFrame->PrincipalChildList().FirstChild();
     if (!rgFrame || rgFrame->GetType() != nsGkAtoms::tableRowGroupFrame)
       return nullptr;
     for (nsIFrame* rowFrame : rgFrame->PrincipalChildList()) {
diff --git a/layout/printing/nsPrintEngine.cpp b/layout/printing/nsPrintEngine.cpp
index bd0d647e33f..76aca13abdd 100644
--- a/layout/printing/nsPrintEngine.cpp
+++ b/layout/printing/nsPrintEngine.cpp
@@ -2238,7 +2238,7 @@ nsPrintEngine::CalcNumPrintablePages(int32_t& aNumPages)
       nsIPageSequenceFrame* pageSequence = po->mPresShell->GetPageSequenceFrame();
       nsIFrame * seqFrame = do_QueryFrame(pageSequence);
       if (seqFrame) {
-        nsIFrame* frame = seqFrame->GetFirstPrincipalChild();
+        nsIFrame* frame = seqFrame->PrincipalChildList().FirstChild();
         while (frame) {
           aNumPages++;
           frame = frame->GetNextSibling();
@@ -2867,7 +2867,7 @@ nsPrintEngine::GetPageRangeForSelection(nsIPageSequenceFrame* aPageSeqFrame,
   // dump all the pages and their pointers
   {
   int32_t pageNum = 1;
-  nsIFrame* child = seqFrame->GetFirstPrincipalChild();
+  nsIFrame* child = seqFrame->PrincipalChildList().FirstChild();
   while (child != nullptr) {
     printf("Page: %d - %p\n", pageNum, child);
     pageNum++;
@@ -2879,7 +2879,7 @@ nsPrintEngine::GetPageRangeForSelection(nsIPageSequenceFrame* aPageSeqFrame,
   // Now that we have the page frames
   // find out what the page numbers are for each frame
   int32_t pageNum = 1;
-  nsIFrame* page = seqFrame->GetFirstPrincipalChild();
+  nsIFrame* page = seqFrame->PrincipalChildList().FirstChild();
   while (page != nullptr) {
     if (page == startPageFrame) {
       aStartPageNum = pageNum;
@@ -3642,7 +3642,7 @@ static void DumpFrames(FILE*                 out,
   NS_ASSERTION(aRendContext, "Pointer is null!");
   NS_ASSERTION(aFrame, "Pointer is null!");
 
-  nsIFrame* child = aFrame->GetFirstPrincipalChild();
+  nsIFrame* child = aFrame->PrincipalChildList().FirstChild();
   while (child != nullptr) {
     for (int32_t i=0;i<aLevel;i++) {
      fprintf(out, "  ");
@@ -3783,7 +3783,7 @@ static void DumpPrintObjectsList(nsTArray<nsPrintObject*> * aDocList)
         if (sqf) {
           break;
         }
-        rootFrame = rootFrame->GetFirstPrincipalChild();
+        rootFrame = rootFrame->PrincipalChildList().FirstChild();
       }
     }
 
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index d4516745024..b9d9dc34c78 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -673,7 +673,7 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
       if (type == nsGkAtoms::tableOuterFrame) {
         // If the frame is an outer table frame then we should get the style
         // from the inner table frame.
-        mInnerFrame = mOuterFrame->GetFirstPrincipalChild();
+        mInnerFrame = mOuterFrame->PrincipalChildList().FirstChild();
         NS_ASSERTION(mInnerFrame, "Outer table must have an inner");
         NS_ASSERTION(!mInnerFrame->GetNextSibling(),
                      "Outer table frames should have just one child, "
@@ -4891,7 +4891,7 @@ nsComputedDOMStyle::GetAbsoluteOffset(mozilla::css::Side aSide)
     // the containing block is the viewport, which _does_ include
     // scrollbars.  We have to do some extra work.
     // the first child in the default frame list is what we want
-    nsIFrame* scrollingChild = container->GetFirstPrincipalChild();
+    nsIFrame* scrollingChild = container->PrincipalChildList().FirstChild();
     nsIScrollableFrame *scrollFrame = do_QueryFrame(scrollingChild);
     if (scrollFrame) {
       scrollbarSizes = scrollFrame->GetActualScrollbarSizes();
diff --git a/layout/svg/SVGTextFrame.cpp b/layout/svg/SVGTextFrame.cpp
index 8a34c1d5df4..5984c4f86b5 100644
--- a/layout/svg/SVGTextFrame.cpp
+++ b/layout/svg/SVGTextFrame.cpp
@@ -439,7 +439,7 @@ static SVGTextFrame*
 FrameIfAnonymousChildReflowed(SVGTextFrame* aFrame)
 {
   NS_PRECONDITION(aFrame, "aFrame must not be null");
-  nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
   if (NS_SUBTREE_DIRTY(kid)) {
     MOZ_ASSERT(false, "should have already reflowed the anonymous block child");
     return nullptr;
@@ -1477,7 +1477,7 @@ TextNodeCorrespondenceRecorder::TraverseAndRecord(nsIFrame* aFrame)
   // Recursively iterate over the frame tree, for frames that correspond
   // to text content elements.
   if (IsTextContentElement(aFrame->GetContent())) {
-    for (nsIFrame* f = aFrame->GetFirstPrincipalChild();
+    for (nsIFrame* f = aFrame->PrincipalChildList().FirstChild();
          f;
          f = f->GetNextSibling()) {
       TraverseAndRecord(f);
@@ -1758,8 +1758,8 @@ private:
 uint32_t
 TextFrameIterator::UndisplayedCharacters() const
 {
-  MOZ_ASSERT(!(mRootFrame->GetFirstPrincipalChild() &&
-               NS_SUBTREE_DIRTY(mRootFrame->GetFirstPrincipalChild())),
+  MOZ_ASSERT(!(mRootFrame->PrincipalChildList().FirstChild() &&
+               NS_SUBTREE_DIRTY(mRootFrame->PrincipalChildList().FirstChild())),
              "should have already reflowed the anonymous block child");
 
   if (!mCurrentFrame) {
@@ -1779,7 +1779,7 @@ TextFrameIterator::Next()
   if (mCurrentFrame) {
     do {
       nsIFrame* next = IsTextContentElement(mCurrentFrame->GetContent()) ?
-                         mCurrentFrame->GetFirstPrincipalChild() :
+                         mCurrentFrame->PrincipalChildList().FirstChild() :
                          nullptr;
       if (next) {
         // Descend into this frame, and accumulate its position.
@@ -3667,7 +3667,7 @@ SVGTextFrame::PaintSVG(gfxContext& aContext,
 {
   DrawTarget& aDrawTarget = *aContext.GetDrawTarget();
 
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return NS_OK;
 
@@ -3808,7 +3808,7 @@ SVGTextFrame::PaintSVG(gfxContext& aContext,
 nsIFrame*
 SVGTextFrame::GetFrameForPoint(const gfxPoint& aPoint)
 {
-  NS_ASSERTION(GetFirstPrincipalChild(), "must have a child frame");
+  NS_ASSERTION(PrincipalChildList().FirstChild(), "must have a child frame");
 
   if (mState & NS_FRAME_IS_NONDISPLAY) {
     // Text frames inside <clipPath> will never have had ReflowSVG called on
@@ -3977,9 +3977,9 @@ SVGBBox
 SVGTextFrame::GetBBoxContribution(const gfx::Matrix &aToBBoxUserspace,
                                   uint32_t aFlags)
 {
-  NS_ASSERTION(GetFirstPrincipalChild(), "must have a child frame");
+  NS_ASSERTION(PrincipalChildList().FirstChild(), "must have a child frame");
   SVGBBox bbox;
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (kid && NS_SUBTREE_DIRTY(kid)) {
     // Return an empty bbox if our kid's subtree is dirty. This may be called
     // in that situation, e.g. when we're building a display list after an
@@ -4837,7 +4837,7 @@ ShiftAnchoredChunk(nsTArray<mozilla::CharPosition>& aCharPositions,
 void
 SVGTextFrame::AdjustChunksForLineBreaks()
 {
-  nsBlockFrame* block = nsLayoutUtils::GetAsBlock(GetFirstPrincipalChild());
+  nsBlockFrame* block = nsLayoutUtils::GetAsBlock(PrincipalChildList().FirstChild());
   NS_ASSERTION(block, "expected block frame");
 
   nsBlockFrame::line_iterator line = block->begin_lines();
@@ -5128,7 +5128,7 @@ SVGTextFrame::DoGlyphPositioning()
   mPositions.Clear();
   RemoveStateBits(NS_STATE_SVG_POSITIONING_DIRTY);
 
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (kid && NS_SUBTREE_DIRTY(kid)) {
     MOZ_ASSERT(false, "should have already reflowed the kid");
     return;
@@ -5197,8 +5197,8 @@ SVGTextFrame::DoGlyphPositioning()
   double adjustment = 0.0;
   mLengthAdjustScaleFactor = 1.0f;
   if (adjustingTextLength) {
-    nscoord frameLength = vertical ? GetFirstPrincipalChild()->GetRect().height
-                                   : GetFirstPrincipalChild()->GetRect().width;
+    nscoord frameLength = vertical ? PrincipalChildList().FirstChild()->GetRect().height
+                                   : PrincipalChildList().FirstChild()->GetRect().width;
     float actualTextLength =
       static_cast<float>(presContext->AppUnitsToGfxUnits(frameLength) * factor);
 
@@ -5330,7 +5330,7 @@ SVGTextFrame::NotifyGlyphMetricsChange()
 void
 SVGTextFrame::UpdateGlyphPositioning()
 {
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid) {
     return;
   }
@@ -5343,7 +5343,7 @@ SVGTextFrame::UpdateGlyphPositioning()
 void
 SVGTextFrame::MaybeReflowAnonymousBlockChild()
 {
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return;
 
@@ -5386,7 +5386,7 @@ SVGTextFrame::DoReflow()
   }
 
   nsPresContext *presContext = PresContext();
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return;
 
diff --git a/layout/svg/SVGTextFrame.h b/layout/svg/SVGTextFrame.h
index 9b423e9d16f..697ada8aa9d 100644
--- a/layout/svg/SVGTextFrame.h
+++ b/layout/svg/SVGTextFrame.h
@@ -291,7 +291,7 @@ public:
 
   virtual nsContainerFrame* GetContentInsertionFrame() override
   {
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
   virtual void BuildDisplayList(nsDisplayListBuilder*   aBuilder,
diff --git a/layout/svg/nsSVGClipPathFrame.cpp b/layout/svg/nsSVGClipPathFrame.cpp
index cdf092d199f..820e530dd93 100644
--- a/layout/svg/nsSVGClipPathFrame.cpp
+++ b/layout/svg/nsSVGClipPathFrame.cpp
@@ -340,7 +340,7 @@ nsSVGClipPathFrame::IsValid()
     nsIAtom *type = kid->GetType();
 
     if (type == nsGkAtoms::svgUseFrame) {
-      for (nsIFrame* grandKid = kid->GetFirstPrincipalChild(); grandKid;
+      for (nsIFrame* grandKid = kid->PrincipalChildList().FirstChild(); grandKid;
            grandKid = grandKid->GetNextSibling()) {
 
         nsIAtom *type = grandKid->GetType();
diff --git a/layout/svg/nsSVGContainerFrame.cpp b/layout/svg/nsSVGContainerFrame.cpp
index c06ae5bfc74..310b4e6b5d7 100644
--- a/layout/svg/nsSVGContainerFrame.cpp
+++ b/layout/svg/nsSVGContainerFrame.cpp
@@ -112,7 +112,7 @@ nsSVGContainerFrame::ReflowSVGNonDisplayText(nsIFrame* aContainer)
                !aContainer->IsFrameOfType(nsIFrame::eSVG),
                "it is wasteful to call ReflowSVGNonDisplayText on a container "
                "frame that is not NS_FRAME_IS_NONDISPLAY");
-  for (nsIFrame* kid = aContainer->GetFirstPrincipalChild(); kid;
+  for (nsIFrame* kid = aContainer->PrincipalChildList().FirstChild(); kid;
        kid = kid->GetNextSibling()) {
     nsIAtom* type = kid->GetType();
     if (type == nsGkAtoms::svgTextFrame) {
diff --git a/layout/svg/nsSVGForeignObjectFrame.cpp b/layout/svg/nsSVGForeignObjectFrame.cpp
index 0e8c3065a3c..7897967e3e6 100644
--- a/layout/svg/nsSVGForeignObjectFrame.cpp
+++ b/layout/svg/nsSVGForeignObjectFrame.cpp
@@ -211,7 +211,7 @@ nsSVGForeignObjectFrame::PaintSVG(gfxContext& aContext,
   if (IsDisabled())
     return NS_OK;
 
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return NS_OK;
 
@@ -294,7 +294,7 @@ nsSVGForeignObjectFrame::GetFrameForPoint(const gfxPoint& aPoint)
   if (IsDisabled() || (GetStateBits() & NS_FRAME_IS_NONDISPLAY))
     return nullptr;
 
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return nullptr;
 
@@ -360,7 +360,7 @@ nsSVGForeignObjectFrame::ReflowSVG()
 
   // Fully mark our kid dirty so that it gets resized if necessary
   // (NS_FRAME_HAS_DIRTY_CHILDREN isn't enough in that case):
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   kid->AddStateBits(NS_FRAME_IS_DIRTY);
 
   // Make sure to not allow interrupts if we're not being reflown as a root:
@@ -510,7 +510,7 @@ void nsSVGForeignObjectFrame::RequestReflow(nsIPresShell::IntrinsicDirty aType)
     // If we haven't had a ReflowSVG() yet, nothing to do.
     return;
 
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return;
 
@@ -527,7 +527,7 @@ nsSVGForeignObjectFrame::DoReflow()
     return;
 
   nsPresContext *presContext = PresContext();
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (!kid)
     return;
 
@@ -571,7 +571,7 @@ nsSVGForeignObjectFrame::GetInvalidRegion()
   MOZ_ASSERT(!NS_SVGDisplayListPaintingEnabled(),
              "Only called by nsDisplayOuterSVG code");
 
-  nsIFrame* kid = GetFirstPrincipalChild();
+  nsIFrame* kid = PrincipalChildList().FirstChild();
   if (kid->HasInvalidFrameInSubtree()) {
     gfxRect r(mRect.x, mRect.y, mRect.width, mRect.height);
     r.Scale(1.0 / nsPresContext::AppUnitsPerCSSPixel());
diff --git a/layout/svg/nsSVGForeignObjectFrame.h b/layout/svg/nsSVGForeignObjectFrame.h
index ef3c9d419e5..3cd3b03bdf8 100644
--- a/layout/svg/nsSVGForeignObjectFrame.h
+++ b/layout/svg/nsSVGForeignObjectFrame.h
@@ -39,7 +39,7 @@ public:
                                      int32_t         aModType) override;
 
   virtual nsContainerFrame* GetContentInsertionFrame() override {
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
   virtual void Reflow(nsPresContext*           aPresContext,
diff --git a/layout/svg/nsSVGMarkerFrame.cpp b/layout/svg/nsSVGMarkerFrame.cpp
index e5471c644c9..46a114e137e 100644
--- a/layout/svg/nsSVGMarkerFrame.cpp
+++ b/layout/svg/nsSVGMarkerFrame.cpp
@@ -95,7 +95,7 @@ nsSVGMarkerFrame::GetCanvasTM()
 static nsIFrame*
 GetAnonymousChildFrame(nsIFrame* aFrame)
 {
-  nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
   MOZ_ASSERT(kid && kid->GetType() == nsGkAtoms::svgMarkerAnonChildFrame,
              "expected to find anonymous child of marker frame");
   return kid;
diff --git a/layout/svg/nsSVGMarkerFrame.h b/layout/svg/nsSVGMarkerFrame.h
index d23fb9f7cbb..a40c09c510b 100644
--- a/layout/svg/nsSVGMarkerFrame.h
+++ b/layout/svg/nsSVGMarkerFrame.h
@@ -76,11 +76,11 @@ public:
 
   virtual nsContainerFrame* GetContentInsertionFrame() override {
     // Any children must be added to our single anonymous inner frame kid.
-    MOZ_ASSERT(GetFirstPrincipalChild() &&
-               GetFirstPrincipalChild()->GetType() ==
+    MOZ_ASSERT(PrincipalChildList().FirstChild() &&
+               PrincipalChildList().FirstChild()->GetType() ==
                  nsGkAtoms::svgMarkerAnonChildFrame,
                "Where is our anonymous child?");
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
   // nsSVGMarkerFrame methods:
diff --git a/layout/svg/nsSVGOuterSVGFrame.cpp b/layout/svg/nsSVGOuterSVGFrame.cpp
index 0c871b1d848..67432849709 100644
--- a/layout/svg/nsSVGOuterSVGFrame.cpp
+++ b/layout/svg/nsSVGOuterSVGFrame.cpp
@@ -384,7 +384,7 @@ nsSVGOuterSVGFrame::Reflow(nsPresContext*           aPresContext,
   SVGSVGElement *svgElem = static_cast<SVGSVGElement*>(mContent);
 
   nsSVGOuterSVGAnonChildFrame *anonKid =
-    static_cast<nsSVGOuterSVGAnonChildFrame*>(GetFirstPrincipalChild());
+    static_cast<nsSVGOuterSVGAnonChildFrame*>(PrincipalChildList().FirstChild());
 
   if (mState & NS_FRAME_FIRST_REFLOW) {
     // Initialize
@@ -423,9 +423,9 @@ nsSVGOuterSVGFrame::Reflow(nsPresContext*           aPresContext,
     // handled in SVGSVGElement::FlushImageTransformInvalidation.
     //
     if (svgElem->HasViewBoxOrSyntheticViewBox()) {
-      nsIFrame* anonChild = GetFirstPrincipalChild();
+      nsIFrame* anonChild = PrincipalChildList().FirstChild();
       anonChild->AddStateBits(NS_FRAME_IS_DIRTY);
-      for (nsIFrame* child = anonChild->GetFirstPrincipalChild(); child;
+      for (nsIFrame* child = anonChild->PrincipalChildList().FirstChild(); child;
            child = child->GetNextSibling()) {
         child->AddStateBits(NS_FRAME_IS_DIRTY);
       }
@@ -528,7 +528,7 @@ nsSVGOuterSVGFrame::UpdateOverflow()
   nsOverflowAreas overflowAreas(rect, rect);
 
   if (!mIsRootContent) {
-    nsIFrame *anonKid = GetFirstPrincipalChild();
+    nsIFrame *anonKid = PrincipalChildList().FirstChild();
     overflowAreas.VisualOverflow().UnionRect(
       overflowAreas.VisualOverflow(),
       anonKid->GetVisualOverflowRect() + anonKid->GetPosition());
@@ -589,7 +589,7 @@ nsDisplayOuterSVG::HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
 
   nsSVGOuterSVGAnonChildFrame *anonKid =
     static_cast<nsSVGOuterSVGAnonChildFrame*>(
-      outerSVGFrame->GetFirstPrincipalChild());
+      outerSVGFrame->PrincipalChildList().FirstChild());
 
   nsIFrame* frame =
     nsSVGUtils::HitTestChildren(anonKid, svgViewportRelativePoint);
@@ -683,7 +683,7 @@ nsSVGOuterSVGFrame::AttributeChanged(int32_t  aNameSpaceID,
       // make sure our cached transform matrix gets (lazily) updated
       mCanvasTM = nullptr;
 
-      nsSVGUtils::NotifyChildrenOfSVGChange(GetFirstPrincipalChild(),
+      nsSVGUtils::NotifyChildrenOfSVGChange(PrincipalChildList().FirstChild(),
                 aAttribute == nsGkAtoms::viewBox ?
                   TRANSFORM_CHANGED | COORD_CONTEXT_CHANGED : TRANSFORM_CHANGED);
 
@@ -825,7 +825,7 @@ nsSVGOuterSVGFrame::NotifyViewportOrTransformChanged(uint32_t aFlags)
     }
   }
 
-  nsSVGUtils::NotifyChildrenOfSVGChange(GetFirstPrincipalChild(), aFlags);
+  nsSVGUtils::NotifyChildrenOfSVGChange(PrincipalChildList().FirstChild(), aFlags);
 }
 
 //----------------------------------------------------------------------
@@ -836,12 +836,12 @@ nsSVGOuterSVGFrame::PaintSVG(gfxContext& aContext,
                              const gfxMatrix& aTransform,
                              const nsIntRect* aDirtyRect)
 {
-  NS_ASSERTION(GetFirstPrincipalChild()->GetType() ==
+  NS_ASSERTION(PrincipalChildList().FirstChild()->GetType() ==
                  nsGkAtoms::svgOuterSVGAnonChildFrame &&
-               !GetFirstPrincipalChild()->GetNextSibling(),
+               !PrincipalChildList().FirstChild()->GetNextSibling(),
                "We should have a single, anonymous, child");
   nsSVGOuterSVGAnonChildFrame *anonKid =
-    static_cast<nsSVGOuterSVGAnonChildFrame*>(GetFirstPrincipalChild());
+    static_cast<nsSVGOuterSVGAnonChildFrame*>(PrincipalChildList().FirstChild());
   return anonKid->PaintSVG(aContext, aTransform, aDirtyRect);
 }
 
@@ -849,14 +849,14 @@ SVGBBox
 nsSVGOuterSVGFrame::GetBBoxContribution(const gfx::Matrix &aToBBoxUserspace,
                                         uint32_t aFlags)
 {
-  NS_ASSERTION(GetFirstPrincipalChild()->GetType() ==
+  NS_ASSERTION(PrincipalChildList().FirstChild()->GetType() ==
                  nsGkAtoms::svgOuterSVGAnonChildFrame &&
-               !GetFirstPrincipalChild()->GetNextSibling(),
+               !PrincipalChildList().FirstChild()->GetNextSibling(),
                "We should have a single, anonymous, child");
   // We must defer to our child so that we don't include our
   // content->PrependLocalTransformsTo() transforms.
   nsSVGOuterSVGAnonChildFrame *anonKid =
-    static_cast<nsSVGOuterSVGAnonChildFrame*>(GetFirstPrincipalChild());
+    static_cast<nsSVGOuterSVGAnonChildFrame*>(PrincipalChildList().FirstChild());
   return anonKid->GetBBoxContribution(aToBBoxUserspace, aFlags);
 }
 
diff --git a/layout/svg/nsSVGOuterSVGFrame.h b/layout/svg/nsSVGOuterSVGFrame.h
index a132885e95c..eadcf3dc3a7 100644
--- a/layout/svg/nsSVGOuterSVGFrame.h
+++ b/layout/svg/nsSVGOuterSVGFrame.h
@@ -96,11 +96,11 @@ public:
 
   virtual nsContainerFrame* GetContentInsertionFrame() override {
     // Any children must be added to our single anonymous inner frame kid.
-    MOZ_ASSERT(GetFirstPrincipalChild() &&
-               GetFirstPrincipalChild()->GetType() ==
+    MOZ_ASSERT(PrincipalChildList().FirstChild() &&
+               PrincipalChildList().FirstChild()->GetType() ==
                  nsGkAtoms::svgOuterSVGAnonChildFrame,
                "Where is our anonymous child?");
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
   virtual bool IsSVGTransformed(Matrix *aOwnTransform,
@@ -108,7 +108,7 @@ public:
     // Our anonymous wrapper performs the transforms. We simply
     // return whether we are transformed here but don't apply the transforms
     // themselves.
-    return GetFirstPrincipalChild()->IsSVGTransformed();
+    return PrincipalChildList().FirstChild()->IsSVGTransformed();
   }
 
   // nsISVGSVGFrame interface:
diff --git a/layout/svg/nsSVGUtils.cpp b/layout/svg/nsSVGUtils.cpp
index c4d6612a9e1..9dafa81fc18 100644
--- a/layout/svg/nsSVGUtils.cpp
+++ b/layout/svg/nsSVGUtils.cpp
@@ -432,7 +432,7 @@ nsSVGUtils::GetUserToCanvasTM(nsIFrame *aFrame)
 void 
 nsSVGUtils::NotifyChildrenOfSVGChange(nsIFrame *aFrame, uint32_t aFlags)
 {
-  nsIFrame *kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame *kid = aFrame->PrincipalChildList().FirstChild();
 
   while (kid) {
     nsISVGChildFrame* SVGFrame = do_QueryFrame(kid);
diff --git a/layout/tables/nsCellMap.cpp b/layout/tables/nsCellMap.cpp
index 66b0167c57f..ebb4868bf3a 100644
--- a/layout/tables/nsCellMap.cpp
+++ b/layout/tables/nsCellMap.cpp
@@ -1569,7 +1569,7 @@ bool nsCellMap::CellsSpanOut(nsTArray<nsTableRowFrame*>& aRows) const
   int32_t numNewRows = aRows.Length();
   for (int32_t rowX = 0; rowX < numNewRows; rowX++) {
     nsIFrame* rowFrame = (nsIFrame *) aRows.ElementAt(rowX);
-    nsIFrame* childFrame = rowFrame->GetFirstPrincipalChild();
+    nsIFrame* childFrame = rowFrame->PrincipalChildList().FirstChild();
     while (childFrame) {
       nsTableCellFrame *cellFrame = do_QueryFrame(childFrame);
       if (cellFrame) {
@@ -1741,7 +1741,7 @@ nsCellMap::ExpandWithRows(nsTableCellMap&             aMap,
   for (int32_t rowX = startRowIndex; rowX <= endRowIndex; rowX++) {
     nsTableRowFrame* rFrame = aRowFrames.ElementAt(newRowIndex);
     // append cells
-    nsIFrame* cFrame = rFrame->GetFirstPrincipalChild();
+    nsIFrame* cFrame = rFrame->PrincipalChildList().FirstChild();
     int32_t colIndex = 0;
     while (cFrame) {
       nsTableCellFrame *cellFrame = do_QueryFrame(cFrame);
@@ -2186,7 +2186,7 @@ nsCellMap::RebuildConsideringRows(nsTableCellMap&             aMap,
     int32_t numNewRows = aRowsToInsert->Length();
     for (int32_t newRowX = 0; newRowX < numNewRows; newRowX++) {
       nsTableRowFrame* rFrame = aRowsToInsert->ElementAt(newRowX);
-      nsIFrame* cFrame = rFrame->GetFirstPrincipalChild();
+      nsIFrame* cFrame = rFrame->PrincipalChildList().FirstChild();
       while (cFrame) {
         nsTableCellFrame *cellFrame = do_QueryFrame(cFrame);
         if (cellFrame) {
diff --git a/layout/tables/nsTableCellFrame.cpp b/layout/tables/nsTableCellFrame.cpp
index 8c4ec50d6f9..f0f2bca90b0 100644
--- a/layout/tables/nsTableCellFrame.cpp
+++ b/layout/tables/nsTableCellFrame.cpp
@@ -699,7 +699,7 @@ nsTableCellFrame::CellHasVisibleContent(nscoord       height,
     return true;
   if (tableFrame->IsBorderCollapse())
     return true;
-  nsIFrame* innerFrame = kidFrame->GetFirstPrincipalChild();
+  nsIFrame* innerFrame = kidFrame->PrincipalChildList().FirstChild();
   while(innerFrame) {
     nsIAtom* frameType = innerFrame->GetType();
     if (nsGkAtoms::textFrame == frameType) {
diff --git a/layout/tables/nsTableCellFrame.h b/layout/tables/nsTableCellFrame.h
index 0ac28e0cf4c..34e367c109e 100644
--- a/layout/tables/nsTableCellFrame.h
+++ b/layout/tables/nsTableCellFrame.h
@@ -94,7 +94,7 @@ public:
 #endif
 
   virtual nsContainerFrame* GetContentInsertionFrame() override {
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
   virtual nsMargin GetUsedMargin() const override;
diff --git a/layout/tables/nsTableColGroupFrame.cpp b/layout/tables/nsTableColGroupFrame.cpp
index f5f1827cac8..4cc4618af32 100644
--- a/layout/tables/nsTableColGroupFrame.cpp
+++ b/layout/tables/nsTableColGroupFrame.cpp
@@ -54,7 +54,7 @@ void nsTableColGroupFrame::ResetColIndices(nsIFrame*       aFirstColGroup,
       }
       nsIFrame* colFrame = aStartColFrame; 
       if (!colFrame || (colIndex != aFirstColIndex)) {
-        colFrame = colGroupFrame->GetFirstPrincipalChild();
+        colFrame = colGroupFrame->PrincipalChildList().FirstChild();
       }
       while (colFrame) {
         if (nsGkAtoms::tableColFrame == colFrame->GetType()) {
diff --git a/layout/tables/nsTableFrame.cpp b/layout/tables/nsTableFrame.cpp
index 6fde5f72d27..69420c3d68e 100644
--- a/layout/tables/nsTableFrame.cpp
+++ b/layout/tables/nsTableFrame.cpp
@@ -988,7 +988,7 @@ nsTableFrame::CollectRows(nsIFrame*                   aFrame,
 {
   NS_PRECONDITION(aFrame, "null frame");
   int32_t numRows = 0;
-  nsIFrame* childFrame = aFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aFrame->PrincipalChildList().FirstChild();
   while (childFrame) {
     aCollection.AppendElement(static_cast<nsTableRowFrame*>(childFrame));
     numRows++;
@@ -1206,7 +1206,7 @@ nsTableFrame::GenericTraversal(nsDisplayListBuilder* aBuilder, nsFrame* aFrame,
   // stacking context, in which case the child won't use its passed-in
   // BorderBackground list anyway. It does affect cell borders though; this
   // lets us get cell borders into the nsTableFrame's BorderBackground list.
-  nsIFrame* kid = aFrame->GetFirstPrincipalChild();
+  nsIFrame* kid = aFrame->PrincipalChildList().FirstChild();
   while (kid) {
     aFrame->BuildDisplayListForChild(aBuilder, kid, aDirtyRect, aLists);
     kid = kid->GetNextSibling();
@@ -1504,7 +1504,7 @@ nsTableFrame::ProcessRowInserted(nscoord aNewBSize)
   for (uint32_t rgIdx = 0; rgIdx < rowGroups.Length(); rgIdx++) {
     nsTableRowGroupFrame* rgFrame = rowGroups[rgIdx];
     NS_ASSERTION(rgFrame, "Must have rgFrame here");
-    nsIFrame* childFrame = rgFrame->GetFirstPrincipalChild();
+    nsIFrame* childFrame = rgFrame->PrincipalChildList().FirstChild();
     // find the row that was inserted first
     while (childFrame) {
       nsTableRowFrame *rowFrame = do_QueryFrame(childFrame);
@@ -2439,7 +2439,7 @@ nsTableFrame::HomogenousInsertFrames(ChildListID     aListID,
       aPrevFrame = nullptr;
       while (pseudoFrame  && (parentContent ==
                               (content = pseudoFrame->GetContent()))) {
-        pseudoFrame = pseudoFrame->GetFirstPrincipalChild();
+        pseudoFrame = pseudoFrame->PrincipalChildList().FirstChild();
       }
       nsCOMPtr<nsIContent> container = content->GetParent();
       if (MOZ_LIKELY(container)) { // XXX need this null-check, see bug 411823.
@@ -2465,7 +2465,7 @@ nsTableFrame::HomogenousInsertFrames(ChildListID     aListID,
           pseudoFrame = kidFrame;
           while (pseudoFrame  && (parentContent ==
                                   (content = pseudoFrame->GetContent()))) {
-            pseudoFrame = pseudoFrame->GetFirstPrincipalChild();
+            pseudoFrame = pseudoFrame->PrincipalChildList().FirstChild();
           }
           int32_t index = container->IndexOf(content);
           if (index > lastIndex && index < newIndex) {
@@ -3923,7 +3923,7 @@ nsTableFrame::GetFrameAtOrBefore(nsIFrame*       aParentFrame,
   // aPriorChildFrame is not of type aChildType, so we need start from
   // the beginnng and find the closest one
   nsIFrame* lastMatchingFrame = nullptr;
-  nsIFrame* childFrame = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = aParentFrame->PrincipalChildList().FirstChild();
   while (childFrame && (childFrame != aPriorChildFrame)) {
     if (aChildType == childFrame->GetType()) {
       lastMatchingFrame = childFrame;
@@ -3940,13 +3940,13 @@ nsTableFrame::DumpRowGroup(nsIFrame* aKidFrame)
   if (!aKidFrame)
     return;
 
-  nsIFrame* cFrame = aKidFrame->GetFirstPrincipalChild();
+  nsIFrame* cFrame = aKidFrame->PrincipalChildList().FirstChild();
   while (cFrame) {
     nsTableRowFrame *rowFrame = do_QueryFrame(cFrame);
     if (rowFrame) {
       printf("row(%d)=%p ", rowFrame->GetRowIndex(),
              static_cast<void*>(rowFrame));
-      nsIFrame* childFrame = cFrame->GetFirstPrincipalChild();
+      nsIFrame* childFrame = cFrame->PrincipalChildList().FirstChild();
       while (childFrame) {
         nsTableCellFrame *cellFrame = do_QueryFrame(childFrame);
         if (cellFrame) {
diff --git a/layout/tables/nsTableOuterFrame.h b/layout/tables/nsTableOuterFrame.h
index eb0453592ca..3b678222524 100644
--- a/layout/tables/nsTableOuterFrame.h
+++ b/layout/tables/nsTableOuterFrame.h
@@ -51,7 +51,7 @@ public:
                            nsIFrame*       aOldFrame) override;
 
   virtual nsContainerFrame* GetContentInsertionFrame() override {
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   }
 
 #ifdef ACCESSIBILITY
diff --git a/layout/tables/nsTableRowFrame.cpp b/layout/tables/nsTableRowFrame.cpp
index bdf2708abce..c159bc93177 100644
--- a/layout/tables/nsTableRowFrame.cpp
+++ b/layout/tables/nsTableRowFrame.cpp
@@ -435,7 +435,7 @@ nscoord nsTableRowFrame::GetRowBaseline(WritingMode aWM)
   nsSize containerSize = GetSize();
   for (nsIFrame* childFrame : mFrames) {
     if (IS_TABLE_CELL(childFrame->GetType())) {
-      nsIFrame* firstKid = childFrame->GetFirstPrincipalChild();
+      nsIFrame* firstKid = childFrame->PrincipalChildList().FirstChild();
       ascent = std::max(ascent,
                         LogicalRect(aWM, firstKid->GetNormalRect(),
                                     containerSize).BEnd(aWM));
@@ -546,7 +546,7 @@ nsTableRowFrame::CalcBSize(const nsHTMLReflowState& aReflowState)
       }
       // bsize may have changed, adjust descent to absorb any excess difference
       nscoord ascent;
-       if (!kidFrame->GetFirstPrincipalChild()->GetFirstPrincipalChild())
+       if (!kidFrame->PrincipalChildList().FirstChild()->PrincipalChildList().FirstChild())
          ascent = desSize.BSize(wm);
        else
          ascent = cellFrame->GetCellBaseline();
@@ -961,7 +961,7 @@ nsTableRowFrame::ReflowChildren(nsPresContext*           aPresContext,
         }
         // bsize may have changed, adjust descent to absorb any excess difference
         nscoord ascent;
-        if (!kidFrame->GetFirstPrincipalChild()->GetFirstPrincipalChild()) {
+        if (!kidFrame->PrincipalChildList().FirstChild()->PrincipalChildList().FirstChild()) {
           ascent = desiredSize.BSize(wm);
         } else {
           ascent = ((nsTableCellFrame *)kidFrame)->GetCellBaseline();
diff --git a/layout/tables/nsTableRowGroupFrame.cpp b/layout/tables/nsTableRowGroupFrame.cpp
index 1efbb643a75..cb8e1a8e639 100644
--- a/layout/tables/nsTableRowGroupFrame.cpp
+++ b/layout/tables/nsTableRowGroupFrame.cpp
@@ -197,7 +197,7 @@ DisplayRows(nsDisplayListBuilder* aBuilder, nsFrame* aFrame,
 
   // No cursor. Traverse children the hard way and build a cursor while we're at it
   nsTableRowGroupFrame::FrameCursorData* cursor = f->SetupRowCursor();
-  kid = f->GetFirstPrincipalChild();
+  kid = f->PrincipalChildList().FirstChild();
   while (kid) {
     f->BuildDisplayListForChild(aBuilder, kid, aDirtyRect, aLists);
 
@@ -1365,7 +1365,7 @@ nsTableRowGroupFrame::Reflow(nsPresContext*           aPresContext,
   // XXXmats The following is just bogus.  We leave it here for now because
   // ReflowChildren should pull up rows from our next-in-flow before returning
   // a Complete status, but doesn't (bug 804888).
-  if (GetNextInFlow() && GetNextInFlow()->GetFirstPrincipalChild()) {
+  if (GetNextInFlow() && GetNextInFlow()->PrincipalChildList().FirstChild()) {
     NS_FRAME_SET_INCOMPLETE(aStatus);
   }
 
diff --git a/layout/xul/BoxObject.cpp b/layout/xul/BoxObject.cpp
index 516397cc69f..50d5e0633e5 100644
--- a/layout/xul/BoxObject.cpp
+++ b/layout/xul/BoxObject.cpp
@@ -382,7 +382,7 @@ BoxObject::GetFirstChild(nsIDOMElement * *aFirstVisibleChild)
   *aFirstVisibleChild = nullptr;
   nsIFrame* frame = GetFrame(false);
   if (!frame) return NS_OK;
-  nsIFrame* firstFrame = frame->GetFirstPrincipalChild();
+  nsIFrame* firstFrame = frame->PrincipalChildList().FirstChild();
   if (!firstFrame) return NS_OK;
   // get the content for the box and query to a dom element
   nsCOMPtr<nsIDOMElement> el = do_QueryInterface(firstFrame->GetContent());
@@ -429,7 +429,7 @@ BoxObject::GetPreviousSibling(nsIFrame* aParentFrame, nsIFrame* aFrame,
                               nsIDOMElement** aResult)
 {
   *aResult = nullptr;
-  nsIFrame* nextFrame = aParentFrame->GetFirstPrincipalChild();
+  nsIFrame* nextFrame = aParentFrame->PrincipalChildList().FirstChild();
   nsIFrame* prevFrame = nullptr;
   while (nextFrame) {
     if (nextFrame == aFrame)
diff --git a/layout/xul/nsBox.cpp b/layout/xul/nsBox.cpp
index 9696a4095ab..4d5bbc78950 100644
--- a/layout/xul/nsBox.cpp
+++ b/layout/xul/nsBox.cpp
@@ -926,7 +926,7 @@ nsBox::GetChildBox(const nsIFrame* aFrame)
 {
   // box layout ends at box-wrapped frames, so don't allow these frames
   // to report child boxes.
-  return aFrame->IsBoxFrame() ? aFrame->GetFirstPrincipalChild() : nullptr;
+  return aFrame->IsBoxFrame() ? aFrame->PrincipalChildList().FirstChild() : nullptr;
 }
 
 /*static*/ nsIFrame*
diff --git a/layout/xul/nsBoxFrame.cpp b/layout/xul/nsBoxFrame.cpp
index 86b6e01bee7..1c488382454 100644
--- a/layout/xul/nsBoxFrame.cpp
+++ b/layout/xul/nsBoxFrame.cpp
@@ -1107,7 +1107,7 @@ nsBoxFrame::AppendFrames(ChildListID     aListID,
 nsBoxFrame::GetContentInsertionFrame()
 {
   if (GetStateBits() & NS_STATE_BOX_WRAPS_KIDS_IN_BLOCK)
-    return GetFirstPrincipalChild()->GetContentInsertionFrame();
+    return PrincipalChildList().FirstChild()->GetContentInsertionFrame();
   return nsContainerFrame::GetContentInsertionFrame();
 }
 
diff --git a/layout/xul/nsMenuBarFrame.cpp b/layout/xul/nsMenuBarFrame.cpp
index a11961c7544..427cd488a69 100644
--- a/layout/xul/nsMenuBarFrame.cpp
+++ b/layout/xul/nsMenuBarFrame.cpp
@@ -185,7 +185,7 @@ nsMenuBarFrame::FindMenuWithShortcut(nsIDOMKeyEvent* aKeyEvent)
   // Find a most preferred accesskey which should be returned.
   nsIFrame* foundMenu = nullptr;
   size_t foundIndex = accessKeys.NoIndex;
-  nsIFrame* currFrame = immediateParent->GetFirstPrincipalChild();
+  nsIFrame* currFrame = immediateParent->PrincipalChildList().FirstChild();
 
   while (currFrame) {
     nsIContent* current = currFrame->GetContent();
diff --git a/layout/xul/nsMenuFrame.cpp b/layout/xul/nsMenuFrame.cpp
index 859289752e2..a906cda50a9 100644
--- a/layout/xul/nsMenuFrame.cpp
+++ b/layout/xul/nsMenuFrame.cpp
@@ -1358,7 +1358,7 @@ nsMenuFrame::SizeToPopup(nsBoxLayoutState& aState, nsSize& aSize)
       GetBorderAndPadding(borderPadding);
 
       // if there is a scroll frame, add the desired width of the scrollbar as well
-      nsIScrollableFrame* scrollFrame = do_QueryFrame(popupFrame->GetFirstPrincipalChild());
+      nsIScrollableFrame* scrollFrame = do_QueryFrame(popupFrame->PrincipalChildList().FirstChild());
       nscoord scrollbarWidth = 0;
       if (scrollFrame) {
         scrollbarWidth =
@@ -1441,7 +1441,7 @@ nsIScrollableFrame* nsMenuFrame::GetScrollTargetFrame()
   nsMenuPopupFrame* popupFrame = GetPopup();
   if (!popupFrame)
     return nullptr;
-  nsIFrame* childFrame = popupFrame->GetFirstPrincipalChild();
+  nsIFrame* childFrame = popupFrame->PrincipalChildList().FirstChild();
   if (childFrame)
     return popupFrame->GetScrollFrame(childFrame);
   return nullptr;
diff --git a/layout/xul/nsMenuPopupFrame.cpp b/layout/xul/nsMenuPopupFrame.cpp
index 80db680d8d8..51cd481d287 100644
--- a/layout/xul/nsMenuPopupFrame.cpp
+++ b/layout/xul/nsMenuPopupFrame.cpp
@@ -236,7 +236,7 @@ nsMenuPopupFrame::EnsureWidget()
 {
   nsView* ourView = GetView();
   if (!ourView->HasWidget()) {
-    NS_ASSERTION(!mGeneratedChildren && !GetFirstPrincipalChild(),
+    NS_ASSERTION(!mGeneratedChildren && !PrincipalChildList().FirstChild(),
                  "Creating widget for MenuPopupFrame with children");
     CreateWidgetForView(ourView);
   }
@@ -1684,7 +1684,7 @@ nsIScrollableFrame* nsMenuPopupFrame::GetScrollFrame(nsIFrame* aStart)
   // try children
   currFrame = aStart;
   do {
-    nsIFrame* childFrame = currFrame->GetFirstPrincipalChild();
+    nsIFrame* childFrame = currFrame->PrincipalChildList().FirstChild();
     nsIScrollableFrame* sf = GetScrollFrame(childFrame);
     if (sf)
       return sf;
diff --git a/layout/xul/nsProgressMeterFrame.cpp b/layout/xul/nsProgressMeterFrame.cpp
index 4b84f83cf95..2d46386f677 100644
--- a/layout/xul/nsProgressMeterFrame.cpp
+++ b/layout/xul/nsProgressMeterFrame.cpp
@@ -135,7 +135,7 @@ nsProgressMeterFrame::AttributeChanged(int32_t aNameSpaceID,
   if (nsGkAtoms::mode == aAttribute ||
       (!undetermined &&
        (nsGkAtoms::value == aAttribute || nsGkAtoms::max == aAttribute))) {
-    nsIFrame* barChild = GetFirstPrincipalChild();
+    nsIFrame* barChild = PrincipalChildList().FirstChild();
     if (!barChild) return NS_OK;
     nsIFrame* remainderChild = barChild->GetNextSibling();
     if (!remainderChild) return NS_OK;
diff --git a/layout/xul/nsRootBoxFrame.cpp b/layout/xul/nsRootBoxFrame.cpp
index c38cf00f4ff..1c0f8cd4f80 100644
--- a/layout/xul/nsRootBoxFrame.cpp
+++ b/layout/xul/nsRootBoxFrame.cpp
@@ -34,7 +34,7 @@ nsIRootBox::GetRootBox(nsIPresShell* aShell)
   }
 
   if (rootFrame) {
-    rootFrame = rootFrame->GetFirstPrincipalChild();
+    rootFrame = rootFrame->PrincipalChildList().FirstChild();
   }
 
   nsIRootBox* rootBox = do_QueryFrame(rootFrame);
diff --git a/layout/xul/nsScrollbarButtonFrame.cpp b/layout/xul/nsScrollbarButtonFrame.cpp
index 88717448675..bc80e0d119c 100644
--- a/layout/xul/nsScrollbarButtonFrame.cpp
+++ b/layout/xul/nsScrollbarButtonFrame.cpp
@@ -233,7 +233,7 @@ nsScrollbarButtonFrame::GetChildWithTag(nsIAtom* atom, nsIFrame* start,
                                         nsIFrame*& result)
 {
   // recursively search our children
-  nsIFrame* childFrame = start->GetFirstPrincipalChild();
+  nsIFrame* childFrame = start->PrincipalChildList().FirstChild();
   while (nullptr != childFrame) 
   {    
     // get the content node
diff --git a/layout/xul/nsXULPopupManager.cpp b/layout/xul/nsXULPopupManager.cpp
index 4b1610f4579..3f31aeb4d7e 100644
--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -2333,7 +2333,7 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
       currFrame = aStart->GetParent()->GetNextSibling();
   }
   else
-    currFrame = immediateParent->GetFirstPrincipalChild();
+    currFrame = immediateParent->PrincipalChildList().FirstChild();
 
   while (currFrame) {
     // See if it's a menu item.
@@ -2343,7 +2343,7 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
     }
     if (currFrameContent->IsXULElement(nsGkAtoms::menugroup) &&
         currFrameContent->GetChildCount() > 0)
-      currFrame = currFrame->GetFirstPrincipalChild();
+      currFrame = currFrame->PrincipalChildList().FirstChild();
     else if (!currFrame->GetNextSibling() &&
              currFrame->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = currFrame->GetParent()->GetNextSibling();
@@ -2351,7 +2351,7 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
       currFrame = currFrame->GetNextSibling();
   }
 
-  currFrame = immediateParent->GetFirstPrincipalChild();
+  currFrame = immediateParent->PrincipalChildList().FirstChild();
 
   // Still don't have anything. Try cycling from the beginning.
   while (currFrame && currFrame != aStart) {
@@ -2362,7 +2362,7 @@ nsXULPopupManager::GetNextMenuItem(nsContainerFrame* aParent,
     }
     if (currFrameContent->IsXULElement(nsGkAtoms::menugroup) &&
         currFrameContent->GetChildCount() > 0)
-      currFrame = currFrame->GetFirstPrincipalChild();
+      currFrame = currFrame->PrincipalChildList().FirstChild();
     else if (!currFrame->GetNextSibling() &&
              currFrame->GetParent()->GetContent()->IsXULElement(nsGkAtoms::menugroup))
       currFrame = currFrame->GetParent()->GetNextSibling();
diff --git a/layout/xul/tree/nsTreeBodyFrame.cpp b/layout/xul/tree/nsTreeBodyFrame.cpp
index 698cd1702b5..777f3655af1 100644
--- a/layout/xul/tree/nsTreeBodyFrame.cpp
+++ b/layout/xul/tree/nsTreeBodyFrame.cpp
@@ -815,7 +815,7 @@ FindScrollParts(nsIFrame* aCurrFrame, nsTreeBodyFrame::ScrollParts* aResult)
     return;
   }
   
-  nsIFrame* child = aCurrFrame->GetFirstPrincipalChild();
+  nsIFrame* child = aCurrFrame->PrincipalChildList().FirstChild();
   while (child &&
          !child->GetContent()->IsRootOfNativeAnonymousSubtree() &&
          (!aResult->mVScrollbar || !aResult->mHScrollbar ||
diff --git a/layout/xul/tree/nsTreeColumns.cpp b/layout/xul/tree/nsTreeColumns.cpp
index ce6b4315d6b..3dbf6adb980 100644
--- a/layout/xul/tree/nsTreeColumns.cpp
+++ b/layout/xul/tree/nsTreeColumns.cpp
@@ -738,7 +738,7 @@ nsTreeColumns::EnsureColumns()
     if (!colFrame)
       return;
 
-    colFrame = colFrame->GetFirstPrincipalChild();
+    colFrame = colFrame->PrincipalChildList().FirstChild();
     if (!colFrame)
       return;
 
diff --git a/widget/cocoa/nsNativeThemeCocoa.mm b/widget/cocoa/nsNativeThemeCocoa.mm
index b2fce8af343..6cc044cb61c 100644
--- a/widget/cocoa/nsNativeThemeCocoa.mm
+++ b/widget/cocoa/nsNativeThemeCocoa.mm
@@ -2087,7 +2087,7 @@ nsNativeThemeCocoa::GetScrollbarPressStates(nsIFrame* aFrame,
   };
 
   // Get the state of any scrollbar buttons in our child frames
-  for (nsIFrame *childFrame = aFrame->GetFirstPrincipalChild(); 
+  for (nsIFrame *childFrame = aFrame->PrincipalChildList().FirstChild();
        childFrame;
        childFrame = childFrame->GetNextSibling()) {
 
diff --git a/widget/nsNativeTheme.cpp b/widget/nsNativeTheme.cpp
index e8502e4c1ea..7eed9d29549 100644
--- a/widget/nsNativeTheme.cpp
+++ b/widget/nsNativeTheme.cpp
@@ -486,7 +486,7 @@ nsNativeTheme::IsFirstTab(nsIFrame* aFrame)
   if (!aFrame)
     return false;
 
-  nsIFrame* first = aFrame->GetParent()->GetFirstPrincipalChild();
+  nsIFrame* first = aFrame->GetParent()->PrincipalChildList().FirstChild();
   while (first) {
     if (first->GetRect().width > 0 &&
         first->GetContent()->IsXULElement(nsGkAtoms::tab))
@@ -518,7 +518,7 @@ nsNativeTheme::IsNextToSelectedTab(nsIFrame* aFrame, int32_t aOffset)
 
   int32_t thisTabIndex = -1, selectedTabIndex = -1;
 
-  nsIFrame* currentTab = aFrame->GetParent()->GetFirstPrincipalChild();
+  nsIFrame* currentTab = aFrame->GetParent()->PrincipalChildList().FirstChild();
   for (int32_t i = 0; currentTab; currentTab = currentTab->GetNextSibling()) {
     if (currentTab->GetRect().width == 0)
       continue;