diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index caa068e4f3f..946e5260a99 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -1213,7 +1213,6 @@ nsGlobalWindow::~nsGlobalWindow()
     }
   }
 
-  mDocument = nullptr;           // Forces Release
   mDoc = nullptr;
 
   NS_ASSERTION(!mArguments, "mArguments wasn't cleaned up properly!");
@@ -1465,9 +1464,7 @@ nsGlobalWindow::FreeInnerObjects()
     mScreen = nullptr;
   }
 
-  if (mDocument) {
-    NS_ASSERTION(mDoc, "Why is mDoc null?");
-
+  if (mDoc) {
     // Remember the document's principal and URI.
     mDocumentPrincipal = mDoc->NodePrincipal();
     mDocumentURI = mDoc->GetDocumentURI();
@@ -1475,7 +1472,6 @@ nsGlobalWindow::FreeInnerObjects()
   }
 
   // Remove our reference to the document and the document principal.
-  mDocument = nullptr;
   mDoc = nullptr;
   mFocusedNode = nullptr;
 
@@ -1647,7 +1643,6 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(nsGlobalWindow)
   // Traverse stuff from nsPIDOMWindow
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mChromeEventHandler)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mParentTarget)
-  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDocument)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mFrameElement)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mFocusedNode)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mAudioContexts)
@@ -1694,7 +1689,6 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsGlobalWindow)
   // Unlink stuff from nsPIDOMWindow
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mChromeEventHandler)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mParentTarget)
-  NS_IMPL_CYCLE_COLLECTION_UNLINK(mDocument)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mFrameElement)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mFocusedNode)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mAudioContexts)
@@ -2144,7 +2138,7 @@ nsGlobalWindow::SetNewDocument(nsIDocument* aDocument,
 
   NS_ASSERTION(!GetCurrentInnerWindow() ||
                GetCurrentInnerWindow()->GetExtantDoc() == mDoc,
-               "Uh, mDocument doesn't match the current inner window "
+               "Uh, mDoc doesn't match the current inner window "
                "document!");
 
   bool wouldReuseInnerWindow = WouldReuseInnerWindow(aDocument);
@@ -2156,7 +2150,7 @@ nsGlobalWindow::SetNewDocument(nsIDocument* aDocument,
     return NS_ERROR_UNEXPECTED;
   }
 
-  nsCOMPtr<nsIDocument> oldDoc(do_QueryInterface(mDocument));
+  nsCOMPtr<nsIDocument> oldDoc = mDoc;
 
   nsIScriptContext *scx = GetContextInternal();
   NS_ENSURE_TRUE(scx, NS_ERROR_NOT_INITIALIZED);
@@ -2171,7 +2165,7 @@ nsGlobalWindow::SetNewDocument(nsIDocument* aDocument,
   }
 #endif
 
-  if (!mDocument) {
+  if (!mDoc) {
     // First document load.
 
     // Get our private root. If it is equal to us, then we need to
@@ -2198,10 +2192,9 @@ nsGlobalWindow::SetNewDocument(nsIDocument* aDocument,
 
   nsresult rv = NS_OK;
 
-  // Set mDocument even if this is an outer window to avoid
+  // Set mDoc even if this is an outer window to avoid
   // having to *always* reach into the inner window to find the
   // document.
-  mDocument = do_QueryInterface(aDocument);
   mDoc = aDocument;
 
 #ifdef DEBUG
@@ -2471,7 +2464,6 @@ nsGlobalWindow::SetNewDocument(nsIDocument* aDocument,
   if (!aState) {
     if (reUseInnerWindow) {
       if (newInnerWindow->mDoc != aDocument) {
-        newInnerWindow->mDocument = do_QueryInterface(aDocument);
         newInnerWindow->mDoc = aDocument;
 
         // We're reusing the inner window for a new document. In this
@@ -2559,12 +2551,12 @@ nsGlobalWindow::PreloadLocalStorage()
 void
 nsGlobalWindow::DispatchDOMWindowCreated()
 {
-  if (!mDoc || !mDocument) {
+  if (!mDoc) {
     return;
   }
 
   // Fire DOMWindowCreated at chrome event listeners
-  nsContentUtils::DispatchChromeEvent(mDoc, mDocument, NS_LITERAL_STRING("DOMWindowCreated"),
+  nsContentUtils::DispatchChromeEvent(mDoc, mDoc, NS_LITERAL_STRING("DOMWindowCreated"),
                                       true /* bubbles */,
                                       false /* not cancellable */);
 
@@ -2606,7 +2598,6 @@ nsGlobalWindow::InnerSetNewDocument(nsIDocument* aDocument)
   }
 #endif
 
-  mDocument = do_QueryInterface(aDocument);
   mDoc = aDocument;
   mFocusedNode = nullptr;
   mLocalStorage = nullptr;
@@ -2702,14 +2693,13 @@ nsGlobalWindow::DetachFromDocShell()
 
   if (currentInner) {
     NS_ASSERTION(mDoc, "Must have doc!");
-    
+
     // Remember the document's principal and URI.
     mDocumentPrincipal = mDoc->NodePrincipal();
     mDocumentURI = mDoc->GetDocumentURI();
     mDocBaseURI = mDoc->GetDocBaseURI();
 
     // Release our document reference
-    mDocument = nullptr;
     mDoc = nullptr;
     mFocusedNode = nullptr;
   }
@@ -3044,8 +3034,7 @@ nsGlobalWindow::PostHandleEvent(nsEventChainPostVisitor& aVisitor)
              aVisitor.mEvent->mFlags.mIsTrusted) {
     // Execute bindingdetached handlers before we tear ourselves
     // down.
-    if (mDocument) {
-      NS_ASSERTION(mDoc, "Must have doc");
+    if (mDoc) {
       mDoc->BindingManager()->ExecuteDetachedHandlers();
     }
     mIsDocumentLoaded = false;
@@ -3230,7 +3219,7 @@ nsPIDOMWindow::MaybeCreateDoc()
 {
   MOZ_ASSERT(!mDoc);
   if (nsIDocShell* docShell = GetDocShell()) {
-    // Note that |document| here is the same thing as our mDocument, but we
+    // Note that |document| here is the same thing as our mDoc, but we
     // don't have to explicitly set the member variable because the docshell
     // has already called SetNewDocument().
     nsCOMPtr<nsIDocument> document = do_GetInterface(docShell);
@@ -3746,9 +3735,8 @@ nsGlobalWindow::GetApplicationCache(nsIDOMOfflineResourceList **aApplicationCach
     nsresult rv = webNav->GetCurrentURI(getter_AddRefs(uri));
     NS_ENSURE_SUCCESS(rv, rv);
 
-    nsCOMPtr<nsIDocument> doc = do_QueryInterface(mDocument);
     nsCOMPtr<nsIURI> manifestURI;
-    nsContentUtils::GetOfflineAppManifest(doc, getter_AddRefs(manifestURI));
+    nsContentUtils::GetOfflineAppManifest(mDoc, getter_AddRefs(manifestURI));
 
     nsRefPtr<nsDOMOfflineResourceList> applicationCache =
       new nsDOMOfflineResourceList(manifestURI, uri, this);
@@ -4558,8 +4546,7 @@ nsGlobalWindow::CheckSecurityWidthAndHeight(int32_t* aWidth, int32_t* aHeight)
 #ifdef MOZ_XUL
   if (!nsContentUtils::IsCallerChrome()) {
     // if attempting to resize the window, hide any open popups
-    nsCOMPtr<nsIDocument> doc(do_QueryInterface(mDocument));
-    nsContentUtils::HidePopupsInDocument(doc);
+    nsContentUtils::HidePopupsInDocument(mDoc);
   }
 #endif
 
@@ -4624,8 +4611,7 @@ nsGlobalWindow::CheckSecurityLeftAndTop(int32_t* aLeft, int32_t* aTop)
   if (!nsContentUtils::IsCallerChrome()) {
 #ifdef MOZ_XUL
     // if attempting to move the window, hide any open popups
-    nsCOMPtr<nsIDocument> doc(do_QueryInterface(mDocument));
-    nsContentUtils::HidePopupsInDocument(doc);
+    nsContentUtils::HidePopupsInDocument(mDoc);
 #endif
 
     nsGlobalWindow* rootWindow =
@@ -4836,8 +4822,7 @@ bool
 nsGlobalWindow::DispatchCustomEvent(const char *aEventName)
 {
   bool defaultActionEnabled = true;
-  nsCOMPtr<nsIDocument> doc(do_QueryInterface(mDocument));
-  nsContentUtils::DispatchTrustedEvent(doc,
+  nsContentUtils::DispatchTrustedEvent(mDoc,
                                        GetOuterWindow(),
                                        NS_ConvertASCIItoUTF16(aEventName),
                                        true, true, &defaultActionEnabled);
@@ -4999,8 +4984,7 @@ nsGlobalWindow::SetFullScreenInternal(bool aFullScreen, bool aRequireTrust)
     // DOM full-screen mode and the user exits full-screen mode with
     // the browser full-screen mode toggle keyboard-shortcut, we'll detect
     // that and leave DOM API full-screen mode too.
-    nsCOMPtr<nsIDocument> doc(do_QueryInterface(mDocument));
-    nsIDocument::ExitFullscreen(doc, /* async */ false);
+    nsIDocument::ExitFullscreen(mDoc, /* async */ false);
   }
 
   if (!mWakeLock && mFullScreen) {
@@ -5554,10 +5538,8 @@ nsGlobalWindow::Focus()
   mDocShell->GetItemType(&itemType);
   if (itemType == nsIDocShellTreeItem::typeChrome &&
       GetPrivateRoot() == static_cast<nsIDOMWindow*>(this) &&
-      mDocument) {
-    nsCOMPtr<nsIDocument> doc(do_QueryInterface(mDocument));
-    NS_ASSERTION(doc, "Bogus doc?");
-    nsIURI* ourURI = doc->GetDocumentURI();
+      mDoc) {
+    nsIURI* ourURI = mDoc->GetDocumentURI();
     if (ourURI) {
       lookForPresShell = !NS_IsAboutBlank(ourURI);
     }
@@ -5580,8 +5562,7 @@ nsGlobalWindow::Focus()
     if (!parentdoc)
       return NS_OK;
 
-    nsCOMPtr<nsIDocument> doc = do_QueryInterface(mDocument);
-    nsIContent* frame = parentdoc->FindContentForSubDocument(doc);
+    nsIContent* frame = parentdoc->FindContentForSubDocument(mDoc);
     nsCOMPtr<nsIDOMElement> frameElement = do_QueryInterface(frame);
     if (frameElement) {
       uint32_t flags = nsIFocusManager::FLAG_NOSCROLL;
@@ -5623,12 +5604,11 @@ nsGlobalWindow::Blur()
 
     // if the root is focused, clear the focus
     nsIFocusManager* fm = nsFocusManager::GetFocusManager();
-    nsCOMPtr<nsIDocument> doc = do_QueryInterface(mDocument);
-    if (fm && mDocument) {
+    if (fm && mDoc) {
       nsCOMPtr<nsIDOMElement> element;
       fm->GetFocusedElementForWindow(this, false, nullptr, getter_AddRefs(element));
       nsCOMPtr<nsIContent> content = do_QueryInterface(element);
-      if (content == doc->GetRootElement())
+      if (content == mDoc->GetRootElement())
         fm->ClearFocus(this);
     }
   }
@@ -6178,7 +6158,7 @@ nsGlobalWindow::DisableExternalCapture()
 }
 
 static
-bool IsPopupBlocked(nsIDOMDocument* aDoc)
+bool IsPopupBlocked(nsIDocument* aDoc)
 {
   nsCOMPtr<nsIPopupWindowManager> pm =
     do_GetService(NS_POPUPWINDOWMANAGER_CONTRACTID);
@@ -6187,15 +6167,13 @@ bool IsPopupBlocked(nsIDOMDocument* aDoc)
     return false;
   }
 
-  bool blocked = true;
-  nsCOMPtr<nsIDocument> doc(do_QueryInterface(aDoc));
-
-  if (doc) {
-    uint32_t permission = nsIPopupWindowManager::ALLOW_POPUP;
-    pm->TestPermission(doc->NodePrincipal(), &permission);
-    blocked = (permission == nsIPopupWindowManager::DENY_POPUP);
+  if (!aDoc) {
+    return true;
   }
-  return blocked;
+
+  uint32_t permission = nsIPopupWindowManager::ALLOW_POPUP;
+  pm->TestPermission(aDoc->NodePrincipal(), &permission);
+  return permission == nsIPopupWindowManager::DENY_POPUP;
 }
 
 /* static */
@@ -6251,7 +6229,7 @@ nsGlobalWindow::CanSetProperty(const char *aPrefName)
 bool
 nsGlobalWindow::PopupWhitelisted()
 {
-  if (!IsPopupBlocked(mDocument))
+  if (!IsPopupBlocked(mDoc))
     return true;
 
   nsCOMPtr<nsIDOMWindow> parent;
@@ -6795,16 +6773,16 @@ PostMessageEvent::Run()
   }
 
   // Create the event
-  nsCOMPtr<nsIDOMDocument> domDoc = do_QueryInterface(targetWindow->mDocument);
-  if (!domDoc)
+  nsIDocument* doc = targetWindow->mDoc;
+  if (!doc)
     return NS_OK;
-  nsCOMPtr<nsIDOMEvent> event;
-  domDoc->CreateEvent(NS_LITERAL_STRING("MessageEvent"),
-                      getter_AddRefs(event));
+
+  ErrorResult res;
+  nsRefPtr<nsDOMEvent> event = doc->CreateEvent(NS_LITERAL_STRING("MessageEvent"), res);
   if (!event)
     return NS_OK;
 
-  nsCOMPtr<nsIDOMMessageEvent> message = do_QueryInterface(event);
+  nsCOMPtr<nsIDOMMessageEvent> message = do_QueryObject(event);
   nsresult rv = message->InitMessageEvent(NS_LITERAL_STRING("message"),
                                           false /* non-bubbling */,
                                           true /* cancelable */,
@@ -8733,11 +8711,10 @@ nsGlobalWindow::UpdateCanvasFocus(bool aFocusChanged, nsIContent* aNewContent)
     return;
 
   nsCOMPtr<nsIPresShell> presShell = docShell->GetPresShell();
-  if (!presShell || !mDocument)
+  if (!presShell || !mDoc)
     return;
 
-  nsCOMPtr<nsIDocument> doc(do_QueryInterface(mDocument));
-  Element *rootElement = doc->GetRootElement();
+  Element *rootElement = mDoc->GetRootElement();
   if (rootElement) {
       if ((mHasFocus || aFocusChanged) &&
           (mFocusedNode == rootElement || aNewContent == rootElement)) {
@@ -8875,8 +8852,8 @@ nsGlobalWindow::GetSessionStorage(nsIDOMStorage ** aSessionStorage)
     *aSessionStorage = nullptr;
 
     nsString documentURI;
-    if (mDocument) {
-      mDocument->GetDocumentURI(documentURI);
+    if (mDoc) {
+      mDoc->GetDocumentURI(documentURI);
     }
 
     // If the document has the sandboxed origin flag set
@@ -8960,8 +8937,8 @@ nsGlobalWindow::GetLocalStorage(nsIDOMStorage ** aLocalStorage)
     }
 
     nsString documentURI;
-    if (mDocument) {
-      mDocument->GetDocumentURI(documentURI);
+    if (mDoc) {
+      mDoc->GetDocumentURI(documentURI);
     }
 
     nsIDocShell* docShell = GetDocShell();
@@ -9116,9 +9093,8 @@ nsGlobalWindow::FireOfflineStatusEvent()
     }
   }
   else {
-    nsCOMPtr<nsIDOMElement> documentElement;
-    mDocument->GetDocumentElement(getter_AddRefs(documentElement));
-    if(documentElement) {        
+    Element* documentElement = mDoc->GetDocumentElement();
+    if(documentElement) {
       eventTarget = documentElement;
     }
   }
@@ -10016,7 +9992,7 @@ nsGlobalWindow::SetTimeoutOrInterval(nsIScriptTimeoutHandler *aHandler,
 
   // If we don't have a document (we could have been unloaded since
   // the call to setTimeout was made), do nothing.
-  if (!mDocument) {
+  if (!mDoc) {
     return NS_OK;
   }
 
@@ -10839,8 +10815,8 @@ nsGlobalWindow::BuildURIfromBase(const char *aURL, nsIURI **aBuiltURI,
 
   // get JSContext
   NS_ASSERTION(scx, "opening window missing its context");
-  NS_ASSERTION(mDocument, "opening window missing its document");
-  if (!scx || !mDocument)
+  NS_ASSERTION(mDoc, "opening window missing its document");
+  if (!scx || !mDoc)
     return NS_ERROR_FAILURE;
 
   nsCOMPtr<nsIDOMChromeWindow> chrome_win = do_QueryObject(this);
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index 9e1b42d7b57..d891dca5956 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -670,8 +670,7 @@ protected:
   // value on both the outer window and the current inner window. Make
   // sure you keep them in sync!
   nsCOMPtr<mozilla::dom::EventTarget> mChromeEventHandler; // strong
-  nsCOMPtr<nsIDOMDocument> mDocument; // strong
-  nsCOMPtr<nsIDocument> mDoc; // strong, for fast access
+  nsCOMPtr<nsIDocument> mDoc; // strong
   // Cache the URI when mDoc is cleared.
   nsCOMPtr<nsIURI> mDocumentURI; // strong
   nsCOMPtr<nsIURI> mDocBaseURI; // strong