diff --git a/content/base/src/Element.cpp b/content/base/src/Element.cpp
index fd1331b6169..81e93c33156 100644
--- a/content/base/src/Element.cpp
+++ b/content/base/src/Element.cpp
@@ -1728,7 +1728,7 @@ Element::DispatchClickEvent(nsPresContext* aPresContext,
     clickCount = sourceMouseEvent->clickCount;
     pressure = sourceMouseEvent->pressure;
     inputSource = sourceMouseEvent->inputSource;
-  } else if (aSourceEvent->eventStructType == NS_KEY_EVENT) {
+  } else if (aSourceEvent->mClass == NS_KEY_EVENT) {
     inputSource = nsIDOMMouseEvent::MOZ_SOURCE_KEYBOARD;
   }
   event.pressure = pressure;
diff --git a/content/base/src/FragmentOrElement.cpp b/content/base/src/FragmentOrElement.cpp
index 64f2150f143..0dac4e4a2e8 100644
--- a/content/base/src/FragmentOrElement.cpp
+++ b/content/base/src/FragmentOrElement.cpp
@@ -814,7 +814,7 @@ nsIContent::PreHandleEvent(EventChainPreVisitor& aVisitor)
     // all the events are allowed even in the native anonymous content..
     nsCOMPtr<nsIContent> t = do_QueryInterface(aVisitor.mEvent->originalTarget);
     NS_ASSERTION(!t || !t->ChromeOnlyAccess() ||
-                 aVisitor.mEvent->eventStructType != NS_MUTATION_EVENT ||
+                 aVisitor.mEvent->mClass != NS_MUTATION_EVENT ||
                  aVisitor.mDOMEvent,
                  "Mutation event dispatched in native anonymous content!?!");
 #endif
diff --git a/content/base/src/nsContentUtils.cpp b/content/base/src/nsContentUtils.cpp
index cd2baae7e22..5171c97fb5d 100644
--- a/content/base/src/nsContentUtils.cpp
+++ b/content/base/src/nsContentUtils.cpp
@@ -4894,7 +4894,7 @@ nsContentUtils::GetAccelKeyCandidates(nsIDOMKeyEvent* aDOMKeyEvent,
   WidgetKeyboardEvent* nativeKeyEvent =
     aDOMKeyEvent->GetInternalNSEvent()->AsKeyboardEvent();
   if (nativeKeyEvent) {
-    NS_ASSERTION(nativeKeyEvent->eventStructType == NS_KEY_EVENT,
+    NS_ASSERTION(nativeKeyEvent->mClass == NS_KEY_EVENT,
                  "wrong type of native event");
     // nsShortcutCandidate::mCharCode is a candidate charCode.
     // nsShoftcutCandidate::mIgnoreShift means the mCharCode should be tried to
diff --git a/content/html/content/src/HTMLCanvasElement.cpp b/content/html/content/src/HTMLCanvasElement.cpp
index 012238a7a70..7eb1c2e880a 100644
--- a/content/html/content/src/HTMLCanvasElement.cpp
+++ b/content/html/content/src/HTMLCanvasElement.cpp
@@ -293,7 +293,7 @@ HTMLCanvasElement::CopyInnerTo(Element* aDest)
 
 nsresult HTMLCanvasElement::PreHandleEvent(EventChainPreVisitor& aVisitor)
 {
-  if (aVisitor.mEvent->eventStructType == NS_MOUSE_EVENT) {
+  if (aVisitor.mEvent->mClass == NS_MOUSE_EVENT) {
     WidgetMouseEventBase* evt = (WidgetMouseEventBase*)aVisitor.mEvent;
     if (mCurrentContext) {
       nsIFrame *frame = GetPrimaryFrame();
diff --git a/content/html/content/src/HTMLInputElement.cpp b/content/html/content/src/HTMLInputElement.cpp
index 25918ec8f1e..43c5523215d 100644
--- a/content/html/content/src/HTMLInputElement.cpp
+++ b/content/html/content/src/HTMLInputElement.cpp
@@ -3251,7 +3251,7 @@ HTMLInputElement::NeedToInitializeEditorForEvent(
   // handled without the editor being initialized.  These events include:
   // mousein/move/out, overflow/underflow, and DOM mutation events.
   if (!IsSingleLineTextControl(false) ||
-      aVisitor.mEvent->eventStructType == NS_MUTATION_EVENT) {
+      aVisitor.mEvent->mClass == NS_MUTATION_EVENT) {
     return false;
   }
 
@@ -4269,9 +4269,9 @@ HTMLInputElement::PostHandleEventForRangeThumb(EventChainPostVisitor& aVisitor)
   MOZ_ASSERT(mType == NS_FORM_INPUT_RANGE);
 
   if (nsEventStatus_eConsumeNoDefault == aVisitor.mEventStatus ||
-      !(aVisitor.mEvent->eventStructType == NS_MOUSE_EVENT ||
-        aVisitor.mEvent->eventStructType == NS_TOUCH_EVENT ||
-        aVisitor.mEvent->eventStructType == NS_KEY_EVENT)) {
+      !(aVisitor.mEvent->mClass == NS_MOUSE_EVENT ||
+        aVisitor.mEvent->mClass == NS_TOUCH_EVENT ||
+        aVisitor.mEvent->mClass == NS_KEY_EVENT)) {
     return;
   }
 
diff --git a/content/xul/content/src/nsXULElement.cpp b/content/xul/content/src/nsXULElement.cpp
index cb6ec7060bb..7be32aea3e8 100644
--- a/content/xul/content/src/nsXULElement.cpp
+++ b/content/xul/content/src/nsXULElement.cpp
@@ -1289,7 +1289,7 @@ nsXULElement::PreHandleEvent(EventChainPreVisitor& aVisitor)
         return NS_OK;
     }
     if (aVisitor.mEvent->message == NS_XUL_COMMAND &&
-        aVisitor.mEvent->eventStructType == NS_INPUT_EVENT &&
+        aVisitor.mEvent->mClass == NS_INPUT_EVENT &&
         aVisitor.mEvent->originalTarget == static_cast<nsIContent*>(this) &&
         tag != nsGkAtoms::command) {
         // Check that we really have an xul command event. That will be handled
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 29ae4786194..3110908786b 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -13494,7 +13494,7 @@ nsGlobalWindow::BeginWindowMove(Event& aMouseDownEvent, Element* aPanel,
 
   WidgetMouseEvent* mouseEvent =
     aMouseDownEvent.GetInternalNSEvent()->AsMouseEvent();
-  if (!mouseEvent || mouseEvent->eventStructType != NS_MOUSE_EVENT) {
+  if (!mouseEvent || mouseEvent->mClass != NS_MOUSE_EVENT) {
     aError.Throw(NS_ERROR_FAILURE);
     return;
   }
diff --git a/dom/events/CompositionEvent.cpp b/dom/events/CompositionEvent.cpp
index 3d105ffa501..c0728215da4 100644
--- a/dom/events/CompositionEvent.cpp
+++ b/dom/events/CompositionEvent.cpp
@@ -17,7 +17,7 @@ CompositionEvent::CompositionEvent(EventTarget* aOwner,
   : UIEvent(aOwner, aPresContext,
             aEvent ? aEvent : new WidgetCompositionEvent(false, 0, nullptr))
 {
-  NS_ASSERTION(mEvent->eventStructType == NS_COMPOSITION_EVENT,
+  NS_ASSERTION(mEvent->mClass == NS_COMPOSITION_EVENT,
                "event type mismatch");
 
   if (aEvent) {
diff --git a/dom/events/DragEvent.cpp b/dom/events/DragEvent.cpp
index a099fdc9d8f..7bcee4653b0 100644
--- a/dom/events/DragEvent.cpp
+++ b/dom/events/DragEvent.cpp
@@ -116,7 +116,7 @@ DragEvent::GetDataTransfer()
   // with the drag. It is initialized when an attempt is made to retrieve it
   // rather that when the event is created to avoid duplicating the data when
   // no listener ever uses it.
-  if (!mEvent || mEvent->eventStructType != NS_DRAG_EVENT) {
+  if (!mEvent || mEvent->mClass != NS_DRAG_EVENT) {
     NS_WARNING("Tried to get dataTransfer from non-drag event!");
     return nullptr;
   }
diff --git a/dom/events/Event.cpp b/dom/events/Event.cpp
index 646dd88ae60..f61fbbe5884 100644
--- a/dom/events/Event.cpp
+++ b/dom/events/Event.cpp
@@ -157,7 +157,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Event)
     tmp->mEvent->target = nullptr;
     tmp->mEvent->currentTarget = nullptr;
     tmp->mEvent->originalTarget = nullptr;
-    switch (tmp->mEvent->eventStructType) {
+    switch (tmp->mEvent->mClass) {
       case NS_MOUSE_EVENT:
       case NS_MOUSE_SCROLL_EVENT:
       case NS_WHEEL_EVENT:
@@ -195,7 +195,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(Event)
     NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mEvent->target)
     NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mEvent->currentTarget)
     NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mEvent->originalTarget)
-    switch (tmp->mEvent->eventStructType) {
+    switch (tmp->mEvent->mClass) {
       case NS_MOUSE_EVENT:
       case NS_MOUSE_SCROLL_EVENT:
       case NS_WHEEL_EVENT:
@@ -525,7 +525,7 @@ Event::SetEventType(const nsAString& aEventTypeArg)
   if (mIsMainThreadEvent) {
     mEvent->typeString.Truncate();
     mEvent->userType =
-      nsContentUtils::GetEventIdAndAtom(aEventTypeArg, mEvent->eventStructType,
+      nsContentUtils::GetEventIdAndAtom(aEventTypeArg, mEvent->mClass,
                                         &(mEvent->message));
   } else {
     mEvent->userType = nullptr;
@@ -661,7 +661,7 @@ Event::GetEventPopupControlState(WidgetEvent* aEvent)
   // check for exceptions:
   PopupControlState abuse = openAbused;
 
-  switch(aEvent->eventStructType) {
+  switch(aEvent->mClass) {
   case NS_EVENT :
     // For these following events only allow popups if they're
     // triggered while handling user input. See
@@ -847,13 +847,13 @@ Event::GetScreenCoords(nsPresContext* aPresContext,
   }
 
   if (!aEvent || 
-       (aEvent->eventStructType != NS_MOUSE_EVENT &&
-        aEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-        aEvent->eventStructType != NS_WHEEL_EVENT &&
-        aEvent->eventStructType != NS_POINTER_EVENT &&
-        aEvent->eventStructType != NS_TOUCH_EVENT &&
-        aEvent->eventStructType != NS_DRAG_EVENT &&
-        aEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT)) {
+       (aEvent->mClass != NS_MOUSE_EVENT &&
+        aEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+        aEvent->mClass != NS_WHEEL_EVENT &&
+        aEvent->mClass != NS_POINTER_EVENT &&
+        aEvent->mClass != NS_TOUCH_EVENT &&
+        aEvent->mClass != NS_DRAG_EVENT &&
+        aEvent->mClass != NS_SIMPLE_GESTURE_EVENT)) {
     return nsIntPoint(0, 0);
   }
 
@@ -903,13 +903,13 @@ Event::GetClientCoords(nsPresContext* aPresContext,
   }
 
   if (!aEvent ||
-      (aEvent->eventStructType != NS_MOUSE_EVENT &&
-       aEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-       aEvent->eventStructType != NS_WHEEL_EVENT &&
-       aEvent->eventStructType != NS_TOUCH_EVENT &&
-       aEvent->eventStructType != NS_DRAG_EVENT &&
-       aEvent->eventStructType != NS_POINTER_EVENT &&
-       aEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT) ||
+      (aEvent->mClass != NS_MOUSE_EVENT &&
+       aEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+       aEvent->mClass != NS_WHEEL_EVENT &&
+       aEvent->mClass != NS_TOUCH_EVENT &&
+       aEvent->mClass != NS_DRAG_EVENT &&
+       aEvent->mClass != NS_POINTER_EVENT &&
+       aEvent->mClass != NS_SIMPLE_GESTURE_EVENT) ||
       !aPresContext ||
       !aEvent->AsGUIEvent()->widget) {
     return aDefaultPoint;
diff --git a/dom/events/EventDispatcher.cpp b/dom/events/EventDispatcher.cpp
index 28acd8290fa..6e82cbe8dac 100644
--- a/dom/events/EventDispatcher.cpp
+++ b/dom/events/EventDispatcher.cpp
@@ -684,7 +684,7 @@ EventDispatcher::CreateEvent(EventTarget* aOwner,
   *aDOMEvent = nullptr;
 
   if (aEvent) {
-    switch(aEvent->eventStructType) {
+    switch(aEvent->mClass) {
     case NS_MUTATION_EVENT:
       return NS_NewDOMMutationEvent(aDOMEvent, aOwner, aPresContext,
                                     aEvent->AsMutationEvent());
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index fb0badd6165..c62f11343e9 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -497,7 +497,7 @@ EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
   WidgetMouseEvent* mouseEvent = aEvent->AsMouseEvent();
   if (aEvent->mFlags.mIsTrusted &&
       ((mouseEvent && mouseEvent->IsReal()) ||
-       aEvent->eventStructType == NS_WHEEL_EVENT) &&
+       aEvent->mClass == NS_WHEEL_EVENT) &&
       !sIsPointerLocked) {
     sLastScreenPoint =
       UIEvent::CalculateScreenPoint(aPresContext, aEvent);
@@ -511,8 +511,8 @@ EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
       ((mouseEvent && mouseEvent->IsReal() &&
         mouseEvent->message != NS_MOUSE_ENTER &&
         mouseEvent->message != NS_MOUSE_EXIT) ||
-       aEvent->eventStructType == NS_WHEEL_EVENT ||
-       aEvent->eventStructType == NS_KEY_EVENT)) {
+       aEvent->mClass == NS_WHEEL_EVENT ||
+       aEvent->mClass == NS_KEY_EVENT)) {
     if (gMouseOrKeyboardEventCounter == 0) {
       nsCOMPtr<nsIObserverService> obs =
         mozilla::services::GetObserverService();
@@ -1089,7 +1089,7 @@ EventStateManager::DispatchCrossProcessEvent(WidgetEvent* aEvent,
     return false;
   }
 
-  switch (aEvent->eventStructType) {
+  switch (aEvent->mClass) {
   case NS_MOUSE_EVENT: {
     return remote->SendRealMouseEvent(*aEvent->AsMouseEvent());
   }
@@ -1156,7 +1156,7 @@ EventStateManager::GetChildProcessOffset(nsFrameLoader* aFrameLoader,
 bool
 CrossProcessSafeEvent(const WidgetEvent& aEvent)
 {
-  switch (aEvent.eventStructType) {
+  switch (aEvent.mClass) {
   case NS_KEY_EVENT:
   case NS_WHEEL_EVENT:
     return true;
@@ -1200,7 +1200,7 @@ EventStateManager::HandleCrossProcessEvent(WidgetEvent* aEvent,
   //
   // NB: the elements of |targets| must be unique, for correctness.
   nsAutoTArray<nsCOMPtr<nsIContent>, 1> targets;
-  if (aEvent->eventStructType != NS_TOUCH_EVENT ||
+  if (aEvent->mClass != NS_TOUCH_EVENT ||
       aEvent->message == NS_TOUCH_START) {
     // If this event only has one target, and it's remote, add it to
     // the array.
@@ -3013,7 +3013,7 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
   case NS_DRAGDROP_ENTER:
   case NS_DRAGDROP_OVER:
     {
-      NS_ASSERTION(aEvent->eventStructType == NS_DRAG_EVENT, "Expected a drag event");
+      NS_ASSERTION(aEvent->mClass == NS_DRAG_EVENT, "Expected a drag event");
 
       nsCOMPtr<nsIDragSession> dragSession = nsContentUtils::GetDragSession();
       if (!dragSession)
@@ -3787,7 +3787,7 @@ EventStateManager::NotifyMouseOut(WidgetMouseEvent* aMouseEvent,
   // hover state itself, and we have optimizations for hover switching between
   // two nearby elements both deep in the DOM tree that would be defeated by
   // switching the hover state to null here.
-  bool isPointer = aMouseEvent->eventStructType == NS_POINTER_EVENT;
+  bool isPointer = aMouseEvent->mClass == NS_POINTER_EVENT;
   if (!aMovingInto && !isPointer) {
     // Unset :hover
     SetContentState(nullptr, NS_EVENT_STATE_HOVER);
@@ -3850,7 +3850,7 @@ EventStateManager::NotifyMouseOver(WidgetMouseEvent* aMouseEvent,
   // DispatchMouseOrPointerEvent() call below, since NotifyMouseOut() resets it, bug 298477.
   nsCOMPtr<nsIContent> lastOverElement = wrapper->mLastOverElement;
 
-  bool isPointer = aMouseEvent->eventStructType == NS_POINTER_EVENT;
+  bool isPointer = aMouseEvent->mClass == NS_POINTER_EVENT;
   
   Maybe<EnterLeaveDispatcher> enterDispatcher;
   if (dispatch) {
diff --git a/dom/events/IMEStateManager.cpp b/dom/events/IMEStateManager.cpp
index d17144619e0..a78c55ac788 100644
--- a/dom/events/IMEStateManager.cpp
+++ b/dom/events/IMEStateManager.cpp
@@ -130,9 +130,9 @@ GetIMEStateSetOpenName(IMEState::Open aOpen)
 }
 
 static const char*
-GetEventStructName(nsEventStructType aEventStructType)
+GetEventClassIDName(EventClassID aEventClassID)
 {
-  switch (aEventStructType) {
+  switch (aEventClassID) {
     case NS_COMPOSITION_EVENT:
       return "NS_COMPOSITION_EVENT";
     case NS_TEXT_EVENT:
@@ -850,16 +850,16 @@ IMEStateManager::DispatchCompositionEvent(nsINode* aEventTargetNode,
 {
   PR_LOG(sISMLog, PR_LOG_ALWAYS,
     ("ISM: IMEStateManager::DispatchCompositionEvent(aNode=0x%p, "
-     "aPresContext=0x%p, aEvent={ eventStructType=%s, message=%s, "
+     "aPresContext=0x%p, aEvent={ mClass=%s, message=%s, "
      " mFlags={ mIsTrusted=%s, mPropagationStopped=%s } })",
      aEventTargetNode, aPresContext,
-     GetEventStructName(aEvent->eventStructType),
+     GetEventClassIDName(aEvent->mClass),
      GetEventMessageName(aEvent->message),
      GetBoolName(aEvent->mFlags.mIsTrusted),
      GetBoolName(aEvent->mFlags.mPropagationStopped)));
 
-  MOZ_ASSERT(aEvent->eventStructType == NS_COMPOSITION_EVENT ||
-             aEvent->eventStructType == NS_TEXT_EVENT);
+  MOZ_ASSERT(aEvent->mClass == NS_COMPOSITION_EVENT ||
+             aEvent->mClass == NS_TEXT_EVENT);
   if (!aEvent->mFlags.mIsTrusted || aEvent->mFlags.mPropagationStopped) {
     return;
   }
diff --git a/dom/events/InputEvent.cpp b/dom/events/InputEvent.cpp
index 13cd371f5d3..131b05a52e6 100644
--- a/dom/events/InputEvent.cpp
+++ b/dom/events/InputEvent.cpp
@@ -16,7 +16,7 @@ InputEvent::InputEvent(EventTarget* aOwner,
   : UIEvent(aOwner, aPresContext,
             aEvent ? aEvent : new InternalEditorInputEvent(false, 0, nullptr))
 {
-  NS_ASSERTION(mEvent->eventStructType == NS_EDITOR_INPUT_EVENT,
+  NS_ASSERTION(mEvent->mClass == NS_EDITOR_INPUT_EVENT,
                "event type mismatch");
 
   if (aEvent) {
diff --git a/dom/events/InternalMutationEvent.h b/dom/events/InternalMutationEvent.h
index b4f1ee2fefc..58f0b2815eb 100644
--- a/dom/events/InternalMutationEvent.h
+++ b/dom/events/InternalMutationEvent.h
@@ -27,7 +27,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_MUTATION_EVENT,
+    MOZ_ASSERT(mClass == NS_MUTATION_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalMutationEvent* result = new InternalMutationEvent(false, message);
     result->AssignMutationEventData(*this, true);
diff --git a/dom/events/KeyboardEvent.cpp b/dom/events/KeyboardEvent.cpp
index 851e3f66f84..351413df3e4 100644
--- a/dom/events/KeyboardEvent.cpp
+++ b/dom/events/KeyboardEvent.cpp
@@ -19,7 +19,7 @@ KeyboardEvent::KeyboardEvent(EventTarget* aOwner,
   , mInitializedByCtor(false)
   , mInitialzedWhichValue(0)
 {
-  NS_ASSERTION(mEvent->eventStructType == NS_KEY_EVENT, "event type mismatch");
+  NS_ASSERTION(mEvent->mClass == NS_KEY_EVENT, "event type mismatch");
 
   if (aEvent) {
     mEventIsInternal = false;
diff --git a/dom/events/MouseEvent.cpp b/dom/events/MouseEvent.cpp
index d09fe1acfee..84c9ed4db6a 100644
--- a/dom/events/MouseEvent.cpp
+++ b/dom/events/MouseEvent.cpp
@@ -69,7 +69,7 @@ MouseEvent::InitMouseEvent(const nsAString& aType,
     UIEvent::InitUIEvent(aType, aCanBubble, aCancelable, aView, aDetail);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  switch(mEvent->eventStructType) {
+  switch(mEvent->mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
@@ -123,7 +123,7 @@ MouseEvent::InitMouseEvent(const nsAString& aType,
                                aButton, aRelatedTarget);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  switch(mEvent->eventStructType) {
+  switch(mEvent->mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
@@ -154,7 +154,7 @@ MouseEvent::Constructor(const GlobalObject& aGlobal,
                     aRv);
   e->SetTrusted(trusted);
 
-  switch (e->mEvent->eventStructType) {
+  switch (e->mEvent->mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
@@ -213,8 +213,7 @@ MouseEvent::GetButton(int16_t* aButton)
 int16_t
 MouseEvent::Button()
 {
-  switch(mEvent->eventStructType)
-  {
+  switch(mEvent->mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
@@ -239,8 +238,7 @@ MouseEvent::GetButtons(uint16_t* aButtons)
 uint16_t
 MouseEvent::Buttons()
 {
-  switch(mEvent->eventStructType)
-  {
+  switch(mEvent->mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
@@ -265,8 +263,7 @@ already_AddRefed<EventTarget>
 MouseEvent::GetRelatedTarget()
 {
   nsCOMPtr<EventTarget> relatedTarget;
-  switch(mEvent->eventStructType)
-  {
+  switch(mEvent->mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
diff --git a/dom/events/PointerEvent.cpp b/dom/events/PointerEvent.cpp
index fad5fe9ff00..156ac64345f 100644
--- a/dom/events/PointerEvent.cpp
+++ b/dom/events/PointerEvent.cpp
@@ -18,7 +18,7 @@ PointerEvent::PointerEvent(EventTarget* aOwner,
   : MouseEvent(aOwner, aPresContext,
                aEvent ? aEvent : new WidgetPointerEvent(false, 0, nullptr))
 {
-  NS_ASSERTION(mEvent->eventStructType == NS_POINTER_EVENT,
+  NS_ASSERTION(mEvent->mClass == NS_POINTER_EVENT,
                "event type mismatch NS_POINTER_EVENT");
 
   WidgetMouseEvent* mouseEvent = mEvent->AsMouseEvent();
diff --git a/dom/events/SimpleGestureEvent.cpp b/dom/events/SimpleGestureEvent.cpp
index b85d33ea7e9..f3d6747da44 100644
--- a/dom/events/SimpleGestureEvent.cpp
+++ b/dom/events/SimpleGestureEvent.cpp
@@ -17,7 +17,8 @@ SimpleGestureEvent::SimpleGestureEvent(EventTarget* aOwner,
                aEvent ? aEvent :
                         new WidgetSimpleGestureEvent(false, 0, nullptr))
 {
-  NS_ASSERTION(mEvent->eventStructType == NS_SIMPLE_GESTURE_EVENT, "event type mismatch");
+  NS_ASSERTION(mEvent->mClass == NS_SIMPLE_GESTURE_EVENT,
+               "event type mismatch");
 
   if (aEvent) {
     mEventIsInternal = false;
diff --git a/dom/events/TextComposition.cpp b/dom/events/TextComposition.cpp
index a78be0585d5..80bfea4b2c3 100644
--- a/dom/events/TextComposition.cpp
+++ b/dom/events/TextComposition.cpp
@@ -110,7 +110,7 @@ TextComposition::NotityUpdateComposition(WidgetGUIEvent* aEvent)
       mCompositionStartOffset = 0;
     }
     mCompositionTargetOffset = mCompositionStartOffset;
-  } else if (aEvent->eventStructType != NS_TEXT_EVENT) {
+  } else if (aEvent->mClass != NS_TEXT_EVENT) {
     return;
   } else {
     mCompositionTargetOffset =
diff --git a/dom/events/UIEvent.cpp b/dom/events/UIEvent.cpp
index 944463d5c12..d9748151bc9 100644
--- a/dom/events/UIEvent.cpp
+++ b/dom/events/UIEvent.cpp
@@ -46,8 +46,7 @@ UIEvent::UIEvent(EventTarget* aOwner,
   
   // Fill mDetail and mView according to the mEvent (widget-generated
   // event) we've got
-  switch(mEvent->eventStructType)
-  {
+  switch(mEvent->mClass) {
     case NS_UI_EVENT:
     {
       mDetail = mEvent->AsUIEvent()->detail;
@@ -119,12 +118,12 @@ UIEvent::GetMovementPoint()
   }
 
   if (!mEvent ||
-      (mEvent->eventStructType != NS_MOUSE_EVENT &&
-       mEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-       mEvent->eventStructType != NS_WHEEL_EVENT &&
-       mEvent->eventStructType != NS_DRAG_EVENT &&
-       mEvent->eventStructType != NS_POINTER_EVENT &&
-       mEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT) ||
+      (mEvent->mClass != NS_MOUSE_EVENT &&
+       mEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+       mEvent->mClass != NS_WHEEL_EVENT &&
+       mEvent->mClass != NS_DRAG_EVENT &&
+       mEvent->mClass != NS_POINTER_EVENT &&
+       mEvent->mClass != NS_SIMPLE_GESTURE_EVENT) ||
        !mEvent->AsGUIEvent()->widget) {
     return nsIntPoint(0, 0);
   }
@@ -297,13 +296,13 @@ nsIntPoint
 UIEvent::GetLayerPoint() const
 {
   if (!mEvent ||
-      (mEvent->eventStructType != NS_MOUSE_EVENT &&
-       mEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-       mEvent->eventStructType != NS_WHEEL_EVENT &&
-       mEvent->eventStructType != NS_POINTER_EVENT &&
-       mEvent->eventStructType != NS_TOUCH_EVENT &&
-       mEvent->eventStructType != NS_DRAG_EVENT &&
-       mEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT) ||
+      (mEvent->mClass != NS_MOUSE_EVENT &&
+       mEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+       mEvent->mClass != NS_WHEEL_EVENT &&
+       mEvent->mClass != NS_POINTER_EVENT &&
+       mEvent->mClass != NS_TOUCH_EVENT &&
+       mEvent->mClass != NS_DRAG_EVENT &&
+       mEvent->mClass != NS_SIMPLE_GESTURE_EVENT) ||
       !mPresContext ||
       mEventIsInternal) {
     return mLayerPoint;
diff --git a/dom/events/UIEvent.h b/dom/events/UIEvent.h
index 2d3d17aa347..fc50af6c427 100644
--- a/dom/events/UIEvent.h
+++ b/dom/events/UIEvent.h
@@ -43,12 +43,12 @@ public:
                                          WidgetEvent* aEvent)
   {
     if (!aEvent ||
-        (aEvent->eventStructType != NS_MOUSE_EVENT &&
-         aEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-         aEvent->eventStructType != NS_WHEEL_EVENT &&
-         aEvent->eventStructType != NS_DRAG_EVENT &&
-         aEvent->eventStructType != NS_POINTER_EVENT &&
-         aEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT)) {
+        (aEvent->mClass != NS_MOUSE_EVENT &&
+         aEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+         aEvent->mClass != NS_WHEEL_EVENT &&
+         aEvent->mClass != NS_DRAG_EVENT &&
+         aEvent->mClass != NS_POINTER_EVENT &&
+         aEvent->mClass != NS_SIMPLE_GESTURE_EVENT)) {
       return nsIntPoint(0, 0);
     }
 
@@ -70,12 +70,12 @@ public:
                                           CSSIntPoint* aDefaultClientPoint)
   {
     if (!aEvent ||
-        (aEvent->eventStructType != NS_MOUSE_EVENT &&
-         aEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-         aEvent->eventStructType != NS_WHEEL_EVENT &&
-         aEvent->eventStructType != NS_DRAG_EVENT &&
-         aEvent->eventStructType != NS_POINTER_EVENT &&
-         aEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT) ||
+        (aEvent->mClass != NS_MOUSE_EVENT &&
+         aEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+         aEvent->mClass != NS_WHEEL_EVENT &&
+         aEvent->mClass != NS_DRAG_EVENT &&
+         aEvent->mClass != NS_POINTER_EVENT &&
+         aEvent->mClass != NS_SIMPLE_GESTURE_EVENT) ||
         !aPresContext ||
         !aEvent->AsGUIEvent()->widget) {
       return aDefaultClientPoint
@@ -132,9 +132,9 @@ public:
 
   virtual uint32_t Which()
   {
-    MOZ_ASSERT(mEvent->eventStructType != NS_KEY_EVENT,
+    MOZ_ASSERT(mEvent->mClass != NS_KEY_EVENT,
                "Key events should override Which()");
-    MOZ_ASSERT(mEvent->eventStructType != NS_MOUSE_EVENT,
+    MOZ_ASSERT(mEvent->mClass != NS_MOUSE_EVENT,
                "Mouse events should override Which()");
     return 0;
   }
diff --git a/dom/ipc/TabParent.cpp b/dom/ipc/TabParent.cpp
index 7950615de04..f1e6fe05fd1 100644
--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -779,7 +779,7 @@ void
 TabParent::MapEventCoordinatesForChildProcess(
   const LayoutDeviceIntPoint& aOffset, WidgetEvent* aEvent)
 {
-  if (aEvent->eventStructType != NS_TOUCH_EVENT) {
+  if (aEvent->mClass != NS_TOUCH_EVENT) {
     aEvent->refPoint = aOffset;
   } else {
     aEvent->refPoint = LayoutDeviceIntPoint();
@@ -1015,7 +1015,7 @@ TabParent::TryCapture(const WidgetGUIEvent& aEvent)
 {
   MOZ_ASSERT(sEventCapturer == this && mEventCaptureDepth > 0);
 
-  if (aEvent.eventStructType != NS_TOUCH_EVENT) {
+  if (aEvent.mClass != NS_TOUCH_EVENT) {
     // Only capture of touch events is implemented, for now.
     return false;
   }
diff --git a/dom/plugins/base/nsPluginInstanceOwner.cpp b/dom/plugins/base/nsPluginInstanceOwner.cpp
index cba3e3c0552..79e2ef65fd7 100644
--- a/dom/plugins/base/nsPluginInstanceOwner.cpp
+++ b/dom/plugins/base/nsPluginInstanceOwner.cpp
@@ -1638,7 +1638,7 @@ nsresult nsPluginInstanceOwner::DispatchKeyToPlugin(nsIDOMEvent* aKeyEvent)
   if (mInstance) {
     WidgetKeyboardEvent* keyEvent =
       aKeyEvent->GetInternalNSEvent()->AsKeyboardEvent();
-    if (keyEvent && keyEvent->eventStructType == NS_KEY_EVENT) {
+    if (keyEvent && keyEvent->mClass == NS_KEY_EVENT) {
       nsEventStatus rv = ProcessEvent(*keyEvent);
       if (nsEventStatus_eConsumeNoDefault == rv) {
         aKeyEvent->PreventDefault();
@@ -1673,7 +1673,7 @@ nsPluginInstanceOwner::ProcessMouseDown(nsIDOMEvent* aMouseEvent)
 
   WidgetMouseEvent* mouseEvent =
     aMouseEvent->GetInternalNSEvent()->AsMouseEvent();
-  if (mouseEvent && mouseEvent->eventStructType == NS_MOUSE_EVENT) {
+  if (mouseEvent && mouseEvent->mClass == NS_MOUSE_EVENT) {
     mLastMouseDownButtonType = mouseEvent->button;
     nsEventStatus rv = ProcessEvent(*mouseEvent);
     if (nsEventStatus_eConsumeNoDefault == rv) {
@@ -1698,7 +1698,7 @@ nsresult nsPluginInstanceOwner::DispatchMouseToPlugin(nsIDOMEvent* aMouseEvent,
 
   WidgetMouseEvent* mouseEvent =
     aMouseEvent->GetInternalNSEvent()->AsMouseEvent();
-  if (mouseEvent && mouseEvent->eventStructType == NS_MOUSE_EVENT) {
+  if (mouseEvent && mouseEvent->mClass == NS_MOUSE_EVENT) {
     nsEventStatus rv = ProcessEvent(*mouseEvent);
     if (nsEventStatus_eConsumeNoDefault == rv) {
       aMouseEvent->PreventDefault();
@@ -1904,7 +1904,7 @@ nsEventStatus nsPluginInstanceOwner::ProcessEvent(const WidgetGUIEvent& anEvent)
   // we can get synthetic events from the EventStateManager... these
   // have no pluginEvent
   NPEvent pluginEvent;
-  if (anEvent.eventStructType == NS_MOUSE_EVENT) {
+  if (anEvent.mClass == NS_MOUSE_EVENT) {
     if (!pPluginEvent) {
       // XXX Should extend this list to synthesize events for more event
       // types
@@ -2011,8 +2011,7 @@ nsEventStatus nsPluginInstanceOwner::ProcessEvent(const WidgetGUIEvent& anEvent)
   XEvent pluginEvent = XEvent();
   pluginEvent.type = 0;
 
-  switch(anEvent.eventStructType)
-    {
+  switch(anEvent.mClass) {
     case NS_MOUSE_EVENT:
       {
         switch (anEvent.message)
@@ -2213,8 +2212,7 @@ nsEventStatus nsPluginInstanceOwner::ProcessEvent(const WidgetGUIEvent& anEvent)
       fm->SetFocus(elem, 0);
     }
   }
-  switch(anEvent.eventStructType)
-    {
+  switch(anEvent.mClass) {
     case NS_MOUSE_EVENT:
       {
         switch (anEvent.message)
diff --git a/gfx/layers/apz/src/APZCTreeManager.cpp b/gfx/layers/apz/src/APZCTreeManager.cpp
index dae66f42b56..b26d90a08b1 100644
--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -693,7 +693,7 @@ APZCTreeManager::ReceiveInputEvent(WidgetInputEvent& aEvent,
 
   MOZ_ASSERT(NS_IsMainThread());
 
-  switch (aEvent.eventStructType) {
+  switch (aEvent.mClass) {
     case NS_TOUCH_EVENT: {
       WidgetTouchEvent& touchEvent = *aEvent.AsTouchEvent();
       MultiTouchInput touchInput(touchEvent);
diff --git a/layout/base/PositionedEventTargeting.cpp b/layout/base/PositionedEventTargeting.cpp
index 1d4407d416a..ddc2145d279 100644
--- a/layout/base/PositionedEventTargeting.cpp
+++ b/layout/base/PositionedEventTargeting.cpp
@@ -74,14 +74,14 @@ static EventRadiusPrefs sMouseEventRadiusPrefs;
 static EventRadiusPrefs sTouchEventRadiusPrefs;
 
 static const EventRadiusPrefs*
-GetPrefsFor(nsEventStructType aEventStructType)
+GetPrefsFor(EventClassID aEventClassID)
 {
   EventRadiusPrefs* prefs = nullptr;
   const char* prefBranch = nullptr;
-  if (aEventStructType == NS_TOUCH_EVENT) {
+  if (aEventClassID == NS_TOUCH_EVENT) {
     prefBranch = "touch";
     prefs = &sTouchEventRadiusPrefs;
-  } else if (aEventStructType == NS_MOUSE_EVENT) {
+  } else if (aEventClassID == NS_MOUSE_EVENT) {
     // Mostly for testing purposes
     prefBranch = "mouse";
     prefs = &sMouseEventRadiusPrefs;
@@ -105,7 +105,7 @@ GetPrefsFor(nsEventStructType aEventStructType)
       Preferences::AddUintVarCache(&prefs->mSideRadii[i], radiusPref.get(), 0);
     }
 
-    if (aEventStructType == NS_MOUSE_EVENT) {
+    if (aEventClassID == NS_MOUSE_EVENT) {
       Preferences::AddBoolVarCache(&prefs->mTouchOnly,
           "ui.mouse.radius.inputSource.touchOnly", true);
     } else {
@@ -363,14 +363,14 @@ FindFrameTargetedByInputEvent(const WidgetGUIEvent* aEvent,
   nsIFrame* target =
     nsLayoutUtils::GetFrameForPoint(aRootFrame, aPointRelativeToRootFrame, flags);
 
-  const EventRadiusPrefs* prefs = GetPrefsFor(aEvent->eventStructType);
+  const EventRadiusPrefs* prefs = GetPrefsFor(aEvent->mClass);
   if (!prefs || !prefs->mEnabled || (target && IsElementClickable(target, nsGkAtoms::body))) {
     return target;
   }
 
   // Do not modify targeting for actual mouse hardware; only for mouse
   // events generated by touch-screen hardware.
-  if (aEvent->eventStructType == NS_MOUSE_EVENT &&
+  if (aEvent->mClass == NS_MOUSE_EVENT &&
       prefs->mTouchOnly &&
       aEvent->AsMouseEvent()->inputSource !=
         nsIDOMMouseEvent::MOZ_SOURCE_TOUCH) {
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 20d6d71c9e4..801f9da3ceb 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -1869,15 +1869,15 @@ nsPoint
 nsLayoutUtils::GetEventCoordinatesRelativeTo(const WidgetEvent* aEvent,
                                              nsIFrame* aFrame)
 {
-  if (!aEvent || (aEvent->eventStructType != NS_MOUSE_EVENT &&
-                  aEvent->eventStructType != NS_MOUSE_SCROLL_EVENT &&
-                  aEvent->eventStructType != NS_WHEEL_EVENT &&
-                  aEvent->eventStructType != NS_DRAG_EVENT &&
-                  aEvent->eventStructType != NS_SIMPLE_GESTURE_EVENT &&
-                  aEvent->eventStructType != NS_POINTER_EVENT &&
-                  aEvent->eventStructType != NS_GESTURENOTIFY_EVENT &&
-                  aEvent->eventStructType != NS_TOUCH_EVENT &&
-                  aEvent->eventStructType != NS_QUERY_CONTENT_EVENT))
+  if (!aEvent || (aEvent->mClass != NS_MOUSE_EVENT &&
+                  aEvent->mClass != NS_MOUSE_SCROLL_EVENT &&
+                  aEvent->mClass != NS_WHEEL_EVENT &&
+                  aEvent->mClass != NS_DRAG_EVENT &&
+                  aEvent->mClass != NS_SIMPLE_GESTURE_EVENT &&
+                  aEvent->mClass != NS_POINTER_EVENT &&
+                  aEvent->mClass != NS_GESTURENOTIFY_EVENT &&
+                  aEvent->mClass != NS_TOUCH_EVENT &&
+                  aEvent->mClass != NS_QUERY_CONTENT_EVENT))
     return nsPoint(NS_UNCONSTRAINEDSIZE, NS_UNCONSTRAINEDSIZE);
 
   return GetEventCoordinatesRelativeTo(aEvent,
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index dad0632cbc6..ece8a625978 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -465,7 +465,7 @@ public:
 
   virtual void HandleEvent(EventChainPostVisitor& aVisitor) MOZ_OVERRIDE
   {
-    if (aVisitor.mPresContext && aVisitor.mEvent->eventStructType != NS_EVENT) {
+    if (aVisitor.mPresContext && aVisitor.mEvent->mClass != NS_EVENT) {
       if (aVisitor.mEvent->message == NS_MOUSE_BUTTON_DOWN ||
           aVisitor.mEvent->message == NS_MOUSE_BUTTON_UP) {
         // Mouse-up and mouse-down events call nsFrame::HandlePress/Release
@@ -6704,7 +6704,7 @@ DispatchPointerFromMouseOrTouch(PresShell* aShell,
                                 nsEventStatus* aStatus)
 {
   uint32_t pointerMessage = 0;
-  if (aEvent->eventStructType == NS_MOUSE_EVENT) {
+  if (aEvent->mClass == NS_MOUSE_EVENT) {
     WidgetMouseEvent* mouseEvent = aEvent->AsMouseEvent();
     // if it is not mouse then it is likely will come as touch event
     if (!mouseEvent->convertToPointer) {
@@ -6736,7 +6736,7 @@ DispatchPointerFromMouseOrTouch(PresShell* aShell,
                      0.0f;
     event.convertToPointer = mouseEvent->convertToPointer = false;
     aShell->HandleEvent(aFrame, &event, aDontRetargetEvents, aStatus);
-  } else if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+  } else if (aEvent->mClass == NS_TOUCH_EVENT) {
     WidgetTouchEvent* touchEvent = aEvent->AsTouchEvent();
     // loop over all touches and dispatch pointer events on each touch
     // copy the event
@@ -6898,8 +6898,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
 
   nsIContent* capturingContent =
     (aEvent->HasMouseEventMessage() ||
-     aEvent->eventStructType == NS_WHEEL_EVENT ? GetCapturingContent() :
-                                                 nullptr);
+     aEvent->mClass == NS_WHEEL_EVENT ? GetCapturingContent() : nullptr);
 
   nsCOMPtr<nsIDocument> retargetEventDoc;
   if (!aDontRetargetEvents) {
@@ -6928,7 +6927,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
       // document that is being captured.
       retargetEventDoc = capturingContent->GetCrossShadowCurrentDoc();
 #ifdef ANDROID
-    } else if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+    } else if (aEvent->mClass == NS_TOUCH_EVENT) {
       retargetEventDoc = GetTouchEventTargetDocument();
 #endif
     }
@@ -6958,7 +6957,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
     }
   }
 
-  if (aEvent->eventStructType == NS_KEY_EVENT &&
+  if (aEvent->mClass == NS_KEY_EVENT &&
       mDocument && mDocument->EventHandlingSuppressed()) {
     if (aEvent->message == NS_KEY_DOWN) {
       mNoDelayedKeyEvents = true;
@@ -6976,7 +6975,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
   if (aEvent->IsUsingCoordinates()) {
     ReleasePointerCaptureCaller releasePointerCaptureCaller;
     if (nsLayoutUtils::AreAsyncAnimationsEnabled() && mDocument) {
-      if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+      if (aEvent->mClass == NS_TOUCH_EVENT) {
         nsIDocument::UnlockPointer();
       }
 
@@ -7057,7 +7056,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
     }
 
     // all touch events except for touchstart use a captured target
-    if (aEvent->eventStructType == NS_TOUCH_EVENT &&
+    if (aEvent->mClass == NS_TOUCH_EVENT &&
         aEvent->message != NS_TOUCH_START) {
       captureRetarget = true;
     }
@@ -7155,7 +7154,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
       } else {
         eventPoint = nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, frame);
       }
-      if (mouseEvent && mouseEvent->eventStructType == NS_MOUSE_EVENT &&
+      if (mouseEvent && mouseEvent->mClass == NS_MOUSE_EVENT &&
           mouseEvent->ignoreRootScrollFrame) {
         flags |= INPUT_IGNORE_ROOT_SCROLL_FRAME;
       }
@@ -7184,7 +7183,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
       }
     }
 
-    if (aEvent->eventStructType == NS_POINTER_EVENT &&
+    if (aEvent->mClass == NS_POINTER_EVENT &&
         aEvent->message != NS_POINTER_DOWN) {
       if (WidgetPointerEvent* pointerEvent = aEvent->AsPointerEvent()) {
         uint32_t pointerId = pointerEvent->pointerId;
@@ -7210,7 +7209,7 @@ PresShell::HandleEvent(nsIFrame* aFrame,
 
     // Suppress mouse event if it's being targeted at an element inside
     // a document which needs events suppressed
-    if (aEvent->eventStructType == NS_MOUSE_EVENT &&
+    if (aEvent->mClass == NS_MOUSE_EVENT &&
         frame->PresContext()->Document()->EventHandlingSuppressed()) {
       if (aEvent->message == NS_MOUSE_BUTTON_DOWN) {
         mNoDelayedMouseEvents = true;
@@ -7756,14 +7755,14 @@ PresShell::HandleEventInternal(WidgetEvent* aEvent, nsEventStatus* aStatus)
     if (NS_SUCCEEDED(rv)) {
       bool wasHandlingKeyBoardEvent =
         nsContentUtils::IsHandlingKeyBoardEvent();
-      if (aEvent->eventStructType == NS_KEY_EVENT) {
+      if (aEvent->mClass == NS_KEY_EVENT) {
         nsContentUtils::SetIsHandlingKeyBoardEvent(true);
       }
       if (aEvent->IsAllowedToDispatchDOMEvent()) {
         MOZ_ASSERT(nsContentUtils::IsSafeToRunScript(),
           "Somebody changed aEvent to cause a DOM event!");
         nsPresShellEventCB eventCB(this);
-        if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+        if (aEvent->mClass == NS_TOUCH_EVENT) {
           DispatchTouchEvent(aEvent, aStatus, &eventCB, touchIsNew);
         } else {
           nsCOMPtr<nsINode> eventTarget = mCurrentEventContent.get();
@@ -7784,8 +7783,8 @@ PresShell::HandleEventInternal(WidgetEvent* aEvent, nsEventStatus* aStatus)
             }
           }
           if (eventTarget) {
-            if (aEvent->eventStructType == NS_COMPOSITION_EVENT ||
-                aEvent->eventStructType == NS_TEXT_EVENT) {
+            if (aEvent->mClass == NS_COMPOSITION_EVENT ||
+                aEvent->mClass == NS_TEXT_EVENT) {
               IMEStateManager::DispatchCompositionEvent(eventTarget,
                 mPresContext, aEvent, aStatus, eventCBPtr);
             } else {
diff --git a/layout/forms/nsNumberControlFrame.cpp b/layout/forms/nsNumberControlFrame.cpp
index 949869a587c..24bac5e3abb 100644
--- a/layout/forms/nsNumberControlFrame.cpp
+++ b/layout/forms/nsNumberControlFrame.cpp
@@ -563,8 +563,7 @@ nsNumberControlFrame::GetNumberControlFrameForSpinButton(nsIFrame* aFrame)
 int32_t
 nsNumberControlFrame::GetSpinButtonForPointerEvent(WidgetGUIEvent* aEvent) const
 {
-  MOZ_ASSERT(aEvent->eventStructType == NS_MOUSE_EVENT,
-             "Unexpected event type");
+  MOZ_ASSERT(aEvent->mClass == NS_MOUSE_EVENT, "Unexpected event type");
 
   if (!mSpinBox) {
     // we don't have a spinner
diff --git a/layout/forms/nsRangeFrame.cpp b/layout/forms/nsRangeFrame.cpp
index 36cb66a4c79..74b36ad74eb 100644
--- a/layout/forms/nsRangeFrame.cpp
+++ b/layout/forms/nsRangeFrame.cpp
@@ -466,8 +466,8 @@ nsRangeFrame::GetValueAsFractionOfRange()
 Decimal
 nsRangeFrame::GetValueAtEventPoint(WidgetGUIEvent* aEvent)
 {
-  MOZ_ASSERT(aEvent->eventStructType == NS_MOUSE_EVENT ||
-             aEvent->eventStructType == NS_TOUCH_EVENT,
+  MOZ_ASSERT(aEvent->mClass == NS_MOUSE_EVENT ||
+             aEvent->mClass == NS_TOUCH_EVENT,
              "Unexpected event type - aEvent->refPoint may be meaningless");
 
   MOZ_ASSERT(mContent->IsHTML(nsGkAtoms::input), "bad cast");
@@ -485,7 +485,7 @@ nsRangeFrame::GetValueAtEventPoint(WidgetGUIEvent* aEvent)
   Decimal range = maximum - minimum;
 
   LayoutDeviceIntPoint absPoint;
-  if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+  if (aEvent->mClass == NS_TOUCH_EVENT) {
     MOZ_ASSERT(aEvent->AsTouchEvent()->touches.Length() == 1,
                "Unexpected number of touches");
     absPoint = LayoutDeviceIntPoint::FromUntyped(
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index a7669b1dcc2..b38ec944287 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -2465,9 +2465,9 @@ nsFrame::HandleEvent(nsPresContext* aPresContext,
     return HandleDrag(aPresContext, aEvent, aEventStatus);
   }
 
-  if ((aEvent->eventStructType == NS_MOUSE_EVENT &&
+  if ((aEvent->mClass == NS_MOUSE_EVENT &&
        aEvent->AsMouseEvent()->button == WidgetMouseEvent::eLeftButton) ||
-      aEvent->eventStructType == NS_TOUCH_EVENT) {
+      aEvent->mClass == NS_TOUCH_EVENT) {
     if (aEvent->message == NS_MOUSE_BUTTON_DOWN || aEvent->message == NS_TOUCH_START) {
       HandlePress(aPresContext, aEvent, aEventStatus);
     } else if (aEvent->message == NS_MOUSE_BUTTON_UP || aEvent->message == NS_TOUCH_END) {
@@ -2679,7 +2679,7 @@ nsFrame::HandlePress(nsPresContext* aPresContext,
   }
 
   NS_ENSURE_ARG_POINTER(aEvent);
-  if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+  if (aEvent->mClass == NS_TOUCH_EVENT) {
     return NS_OK;
   }
 
@@ -3073,7 +3073,8 @@ NS_IMETHODIMP nsFrame::HandleDrag(nsPresContext* aPresContext,
                                   WidgetGUIEvent* aEvent,
                                   nsEventStatus* aEventStatus)
 {
-  MOZ_ASSERT(aEvent->eventStructType == NS_MOUSE_EVENT, "HandleDrag can only handle mouse event");
+  MOZ_ASSERT(aEvent->mClass == NS_MOUSE_EVENT,
+             "HandleDrag can only handle mouse event");
 
   bool selectable;
   IsSelectable(&selectable, nullptr);
@@ -3208,7 +3209,7 @@ NS_IMETHODIMP nsFrame::HandleRelease(nsPresContext* aPresContext,
                                      WidgetGUIEvent* aEvent,
                                      nsEventStatus* aEventStatus)
 {
-  if (aEvent->eventStructType != NS_MOUSE_EVENT) {
+  if (aEvent->mClass != NS_MOUSE_EVENT) {
     return NS_OK;
   }
 
diff --git a/layout/xul/nsResizerFrame.cpp b/layout/xul/nsResizerFrame.cpp
index 8e52ac23423..a14a5ca811e 100644
--- a/layout/xul/nsResizerFrame.cpp
+++ b/layout/xul/nsResizerFrame.cpp
@@ -64,8 +64,8 @@ nsResizerFrame::HandleEvent(nsPresContext* aPresContext,
   switch (aEvent->message) {
     case NS_TOUCH_START:
     case NS_MOUSE_BUTTON_DOWN: {
-      if (aEvent->eventStructType == NS_TOUCH_EVENT ||
-          (aEvent->eventStructType == NS_MOUSE_EVENT &&
+      if (aEvent->mClass == NS_TOUCH_EVENT ||
+          (aEvent->mClass == NS_MOUSE_EVENT &&
            aEvent->AsMouseEvent()->button == WidgetMouseEvent::eLeftButton)) {
         nsCOMPtr<nsIBaseWindow> window;
         nsIPresShell* presShell = aPresContext->GetPresShell();
@@ -129,8 +129,8 @@ nsResizerFrame::HandleEvent(nsPresContext* aPresContext,
 
   case NS_TOUCH_END:
   case NS_MOUSE_BUTTON_UP: {
-    if (aEvent->eventStructType == NS_TOUCH_EVENT ||
-        (aEvent->eventStructType == NS_MOUSE_EVENT &&
+    if (aEvent->mClass == NS_TOUCH_EVENT ||
+        (aEvent->mClass == NS_MOUSE_EVENT &&
          aEvent->AsMouseEvent()->button == WidgetMouseEvent::eLeftButton)) {
       // we're done tracking.
       mTrackingMouseMove = false;
diff --git a/layout/xul/nsSliderFrame.cpp b/layout/xul/nsSliderFrame.cpp
index ef98e9f9a3d..20d3f727259 100644
--- a/layout/xul/nsSliderFrame.cpp
+++ b/layout/xul/nsSliderFrame.cpp
@@ -470,7 +470,7 @@ nsSliderFrame::HandleEvent(nsPresContext* aPresContext,
             isMouseOutsideThumb = true;
         }
       }
-      if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+      if (aEvent->mClass == NS_TOUCH_EVENT) {
         *aEventStatus = nsEventStatus_eConsumeNoDefault;
       }
       if (isMouseOutsideThumb)
@@ -523,7 +523,7 @@ nsSliderFrame::HandleEvent(nsPresContext* aPresContext,
     NS_ENSURE_TRUE(weakFrame.IsAlive(), NS_OK);
 
     DragThumb(true);
-    if (aEvent->eventStructType == NS_TOUCH_EVENT) {
+    if (aEvent->mClass == NS_TOUCH_EVENT) {
       *aEventStatus = nsEventStatus_eConsumeNoDefault;
     }
 
diff --git a/layout/xul/nsXULPopupManager.cpp b/layout/xul/nsXULPopupManager.cpp
index c2edb9ca872..fe69837737b 100644
--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -546,9 +546,9 @@ nsXULPopupManager::InitTriggerEvent(nsIDOMEvent* aEvent, nsIContent* aPopup,
             return;
           nsIFrame* rootDocumentRootFrame = rootDocPresContext->
               PresShell()->FrameManager()->GetRootFrame();
-          if ((event->eventStructType == NS_MOUSE_EVENT || 
-               event->eventStructType == NS_MOUSE_SCROLL_EVENT ||
-               event->eventStructType == NS_WHEEL_EVENT) &&
+          if ((event->mClass == NS_MOUSE_EVENT || 
+               event->mClass == NS_MOUSE_SCROLL_EVENT ||
+               event->mClass == NS_WHEEL_EVENT) &&
                !event->AsGUIEvent()->widget) {
             // no widget, so just use the client point if available
             nsCOMPtr<nsIDOMMouseEvent> mouseEvent = do_QueryInterface(aEvent);
diff --git a/layout/xul/tree/nsTreeBodyFrame.cpp b/layout/xul/tree/nsTreeBodyFrame.cpp
index 582dc88b2fa..bb73df13789 100644
--- a/layout/xul/tree/nsTreeBodyFrame.cpp
+++ b/layout/xul/tree/nsTreeBodyFrame.cpp
@@ -2546,7 +2546,7 @@ nsTreeBodyFrame::GetCursor(const nsPoint& aPoint,
 
 static uint32_t GetDropEffect(WidgetGUIEvent* aEvent)
 {
-  NS_ASSERTION(aEvent->eventStructType == NS_DRAG_EVENT, "wrong event type");
+  NS_ASSERTION(aEvent->mClass == NS_DRAG_EVENT, "wrong event type");
   WidgetDragEvent* dragEvent = aEvent->AsDragEvent();
   nsContentUtils::SetDataTransferInEvent(dragEvent);
 
@@ -2724,7 +2724,7 @@ nsTreeBodyFrame::HandleEvent(nsPresContext* aPresContext,
       rv = mView->GetParentIndex(parentIndex, &parentIndex);
     }
 
-    NS_ASSERTION(aEvent->eventStructType == NS_DRAG_EVENT, "wrong event type");
+    NS_ASSERTION(aEvent->mClass == NS_DRAG_EVENT, "wrong event type");
     WidgetDragEvent* dragEvent = aEvent->AsDragEvent();
     nsContentUtils::SetDataTransferInEvent(dragEvent);
 
diff --git a/widget/BasicEvents.h b/widget/BasicEvents.h
index 0b869d4f283..e7e3496b98f 100644
--- a/widget/BasicEvents.h
+++ b/widget/BasicEvents.h
@@ -17,12 +17,13 @@
 #include "nsString.h"
 #include "Units.h"
 
+namespace mozilla {
+
 /******************************************************************************
- * Event Struct Types
- *
- * TODO: Move it into mozilla namespace.
+ * Event Class ID
  ******************************************************************************/
-enum nsEventStructType
+typedef uint8_t EventClassIDType;
+enum EventClassID MOZ_ENUM_TYPE(EventClassIDType)
 {
   // BasicEvents.h
   NS_EVENT,                          // WidgetEvent
@@ -70,6 +71,8 @@ enum nsEventStructType
   NS_MUTATION_EVENT                  // InternalMutationEvent
 };
 
+} // namespace mozilla
+
 /******************************************************************************
  * Messages
  *
@@ -621,10 +624,14 @@ struct EventFlags : public BaseEventFlags
 class WidgetEvent
 {
 protected:
-  WidgetEvent(bool aIsTrusted, uint32_t aMessage,
-              nsEventStructType aStructType) :
-    eventStructType(aStructType), message(aMessage), refPoint(0, 0),
-    lastRefPoint(0, 0), time(0), timeStamp(TimeStamp::Now()), userType(0)
+  WidgetEvent(bool aIsTrusted, uint32_t aMessage, EventClassID aEventClassID)
+    : mClass(aEventClassID)
+    , message(aMessage)
+    , refPoint(0, 0)
+    , lastRefPoint(0, 0)
+    , time(0)
+    , timeStamp(TimeStamp::Now())
+    , userType(nullptr)
   {
     MOZ_COUNT_CTOR(WidgetEvent);
     mFlags.Clear();
@@ -639,9 +646,14 @@ protected:
   }
 
 public:
-  WidgetEvent(bool aIsTrusted, uint32_t aMessage) :
-    eventStructType(NS_EVENT), message(aMessage), refPoint(0, 0),
-    lastRefPoint(0, 0), time(0), timeStamp(TimeStamp::Now()), userType(0)
+  WidgetEvent(bool aIsTrusted, uint32_t aMessage)
+    : mClass(NS_EVENT)
+    , message(aMessage)
+    , refPoint(0, 0)
+    , lastRefPoint(0, 0)
+    , time(0)
+    , timeStamp(TimeStamp::Now())
+    , userType(nullptr)
   {
     MOZ_COUNT_CTOR(WidgetEvent);
     mFlags.Clear();
@@ -663,7 +675,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const
   {
-    MOZ_ASSERT(eventStructType == NS_EVENT,
+    MOZ_ASSERT(mClass == NS_EVENT,
                "Duplicate() must be overridden by sub class");
     WidgetEvent* result = new WidgetEvent(false, message);
     result->AssignEventData(*this, true);
@@ -671,8 +683,7 @@ public:
     return result;
   }
 
-  // See event struct types
-  nsEventStructType eventStructType;
+  EventClassID mClass;
   // See GUI MESSAGES,
   uint32_t message;
   // Relative to the widget of the event, or if there is no widget then it is
@@ -701,10 +712,7 @@ public:
 
   void AssignEventData(const WidgetEvent& aEvent, bool aCopyTargets)
   {
-    // eventStructType should be initialized with the constructor.
-    // However, NS_SVGZOOM_EVENT and NS_SMIL_TIME_EVENT are set after that.
-    // Therefore, we need to copy eventStructType here.
-    eventStructType = aEvent.eventStructType;
+    // mClass should be initialized with the constructor.
     // message should be initialized with the constructor.
     refPoint = aEvent.refPoint;
     // lastRefPoint doesn't need to be copied.
@@ -837,9 +845,9 @@ class WidgetGUIEvent : public WidgetEvent
 {
 protected:
   WidgetGUIEvent(bool aIsTrusted, uint32_t aMessage, nsIWidget* aWidget,
-                 nsEventStructType aStructType) :
-    WidgetEvent(aIsTrusted, aMessage, aStructType),
-    widget(aWidget)
+                 EventClassID aEventClassID)
+    : WidgetEvent(aIsTrusted, aMessage, aEventClassID)
+    , widget(aWidget)
   {
   }
 
@@ -858,7 +866,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_GUI_EVENT,
+    MOZ_ASSERT(mClass == NS_GUI_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetGUIEvent* result = new WidgetGUIEvent(false, message, nullptr);
@@ -997,9 +1005,9 @@ class WidgetInputEvent : public WidgetGUIEvent
 {
 protected:
   WidgetInputEvent(bool aIsTrusted, uint32_t aMessage, nsIWidget* aWidget,
-                   nsEventStructType aStructType) :
-    WidgetGUIEvent(aIsTrusted, aMessage, aWidget, aStructType),
-    modifiers(0)
+                   EventClassID aEventClassID)
+    : WidgetGUIEvent(aIsTrusted, aMessage, aWidget, aEventClassID)
+    , modifiers(0)
   {
   }
 
@@ -1018,7 +1026,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_INPUT_EVENT,
+    MOZ_ASSERT(mClass == NS_INPUT_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetInputEvent* result = new WidgetInputEvent(false, message, nullptr);
@@ -1146,15 +1154,15 @@ protected:
   }
 
   InternalUIEvent(bool aIsTrusted, uint32_t aMessage, nsIWidget* aWidget,
-                  nsEventStructType aStructType)
-    : WidgetGUIEvent(aIsTrusted, aMessage, aWidget, aStructType)
+                  EventClassID aEventClassID)
+    : WidgetGUIEvent(aIsTrusted, aMessage, aWidget, aEventClassID)
     , detail(0)
   {
   }
 
   InternalUIEvent(bool aIsTrusted, uint32_t aMessage,
-                  nsEventStructType aStructType)
-    : WidgetGUIEvent(aIsTrusted, aMessage, nullptr, aStructType)
+                  EventClassID aEventClassID)
+    : WidgetGUIEvent(aIsTrusted, aMessage, nullptr, aEventClassID)
     , detail(0)
   {
   }
@@ -1170,7 +1178,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_UI_EVENT,
+    MOZ_ASSERT(mClass == NS_UI_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalUIEvent* result = new InternalUIEvent(false, message);
     result->AssignUIEventData(*this, true);
diff --git a/widget/ContentEvents.h b/widget/ContentEvents.h
index 61875e0d774..58a6555ee65 100644
--- a/widget/ContentEvents.h
+++ b/widget/ContentEvents.h
@@ -47,7 +47,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_SCROLLPORT_EVENT,
+    MOZ_ASSERT(mClass == NS_SCROLLPORT_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     InternalScrollPortEvent* result =
@@ -88,7 +88,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_SCROLLAREA_EVENT,
+    MOZ_ASSERT(mClass == NS_SCROLLAREA_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     InternalScrollAreaEvent* result =
@@ -129,7 +129,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_FORM_EVENT,
+    MOZ_ASSERT(mClass == NS_FORM_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalFormEvent* result = new InternalFormEvent(false, message);
     result->AssignFormEventData(*this, true);
@@ -166,7 +166,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_CLIPBOARD_EVENT,
+    MOZ_ASSERT(mClass == NS_CLIPBOARD_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalClipboardEvent* result = new InternalClipboardEvent(false, message);
     result->AssignClipboardEventData(*this, true);
@@ -202,7 +202,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_FOCUS_EVENT,
+    MOZ_ASSERT(mClass == NS_FOCUS_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalFocusEvent* result = new InternalFocusEvent(false, message);
     result->AssignFocusEventData(*this, true);
@@ -247,7 +247,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_TRANSITION_EVENT,
+    MOZ_ASSERT(mClass == NS_TRANSITION_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalTransitionEvent* result =
       new InternalTransitionEvent(false, message);
@@ -292,7 +292,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_ANIMATION_EVENT,
+    MOZ_ASSERT(mClass == NS_ANIMATION_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalAnimationEvent* result = new InternalAnimationEvent(false, message);
     result->AssignAnimationEventData(*this, true);
@@ -332,7 +332,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_SVGZOOM_EVENT,
+    MOZ_ASSERT(mClass == NS_SVGZOOM_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     InternalSVGZoomEvent* result = new InternalSVGZoomEvent(false, message);
@@ -369,7 +369,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_SMIL_TIME_EVENT,
+    MOZ_ASSERT(mClass == NS_SMIL_TIME_EVENT,
                "Duplicate() must be overridden by sub class");
     InternalSMILTimeEvent* result = new InternalSMILTimeEvent(false, message);
     result->AssignSMILTimeEventData(*this, true);
diff --git a/widget/MiscEvents.h b/widget/MiscEvents.h
index a54985c9648..ebc56d58319 100644
--- a/widget/MiscEvents.h
+++ b/widget/MiscEvents.h
@@ -110,7 +110,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_COMMAND_EVENT,
+    MOZ_ASSERT(mClass == NS_COMMAND_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetCommandEvent* result =
@@ -153,7 +153,7 @@ public:
   {
     // NOTE: PluginEvent has to be dispatched to nsIFrame::HandleEvent().
     //       So, this event needs to support Duplicate().
-    MOZ_ASSERT(eventStructType == NS_PLUGIN_EVENT,
+    MOZ_ASSERT(mClass == NS_PLUGIN_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetPluginEvent* result = new WidgetPluginEvent(false, message, nullptr);
diff --git a/widget/MouseEvents.h b/widget/MouseEvents.h
index 8a306749229..bd111e3117d 100644
--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -78,10 +78,12 @@ protected:
   }
 
   WidgetMouseEventBase(bool aIsTrusted, uint32_t aMessage, nsIWidget* aWidget,
-                       nsEventStructType aStructType) :
-    WidgetInputEvent(aIsTrusted, aMessage, aWidget, aStructType),
-    button(0), buttons(0), pressure(0),
-    inputSource(nsIDOMMouseEvent::MOZ_SOURCE_MOUSE)
+                       EventClassID aEventClassID)
+    : WidgetInputEvent(aIsTrusted, aMessage, aWidget, aEventClassID)
+    , button(0)
+    , buttons(0)
+    , pressure(0)
+    , inputSource(nsIDOMMouseEvent::MOZ_SOURCE_MOUSE)
  {
  }
 
@@ -188,10 +190,14 @@ protected:
   }
 
   WidgetMouseEvent(bool aIsTrusted, uint32_t aMessage, nsIWidget* aWidget,
-                   nsEventStructType aStructType, reasonType aReason) :
-    WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, aStructType),
-    acceptActivation(false), ignoreRootScrollFrame(false),
-    reason(aReason), context(eNormal), exit(eChild), clickCount(0)
+                   EventClassID aEventClassID, reasonType aReason)
+    : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, aEventClassID)
+    , acceptActivation(false)
+    , ignoreRootScrollFrame(false)
+    , reason(aReason)
+    , context(eNormal)
+    , exit(eChild)
+    , clickCount(0)
   {
     switch (aMessage) {
       case NS_MOUSEENTER:
@@ -239,7 +245,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_MOUSE_EVENT,
+    MOZ_ASSERT(mClass == NS_MOUSE_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetMouseEvent* result =
@@ -311,7 +317,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_DRAG_EVENT,
+    MOZ_ASSERT(mClass == NS_DRAG_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetDragEvent* result = new WidgetDragEvent(false, message, nullptr);
@@ -370,7 +376,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_MOUSE_SCROLL_EVENT,
+    MOZ_ASSERT(mClass == NS_MOUSE_SCROLL_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetMouseScrollEvent* result =
@@ -432,7 +438,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_WHEEL_EVENT,
+    MOZ_ASSERT(mClass == NS_WHEEL_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetWheelEvent* result = new WidgetWheelEvent(false, message, nullptr);
@@ -575,7 +581,7 @@ public:
     , height(0)
     , isPrimary(true)
   {
-    eventStructType = NS_POINTER_EVENT;
+    mClass = NS_POINTER_EVENT;
     UpdateFlags();
   }
 
@@ -599,7 +605,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_POINTER_EVENT,
+    MOZ_ASSERT(mClass == NS_POINTER_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetPointerEvent* result =
diff --git a/widget/TextEvents.h b/widget/TextEvents.h
index 5e4d7815153..bc69695f01f 100644
--- a/widget/TextEvents.h
+++ b/widget/TextEvents.h
@@ -99,7 +99,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_KEY_EVENT,
+    MOZ_ASSERT(mClass == NS_KEY_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetKeyboardEvent* result =
@@ -232,7 +232,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_TEXT_EVENT,
+    MOZ_ASSERT(mClass == NS_TEXT_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetTextEvent* result = new WidgetTextEvent(false, message, nullptr);
@@ -313,7 +313,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_COMPOSITION_EVENT,
+    MOZ_ASSERT(mClass == NS_COMPOSITION_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetCompositionEvent* result =
@@ -566,7 +566,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_EDITOR_INPUT_EVENT,
+    MOZ_ASSERT(mClass == NS_EDITOR_INPUT_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     InternalEditorInputEvent* result =
diff --git a/widget/TouchEvents.h b/widget/TouchEvents.h
index bbf0f56b644..e0b71cf35f4 100644
--- a/widget/TouchEvents.h
+++ b/widget/TouchEvents.h
@@ -50,7 +50,7 @@ public:
     //     in PreHandleEvent() and not to dispatch as a DOM event into the DOM
     //     tree like ContentQueryEvent.  Then, this event doesn't need to
     //     support Duplicate().
-    MOZ_ASSERT(eventStructType == NS_GESTURENOTIFY_EVENT,
+    MOZ_ASSERT(mClass == NS_GESTURENOTIFY_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetGestureNotifyEvent* result =
@@ -116,7 +116,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_SIMPLE_GESTURE_EVENT,
+    MOZ_ASSERT(mClass == NS_SIMPLE_GESTURE_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetSimpleGestureEvent* result =
@@ -190,7 +190,7 @@ public:
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(eventStructType == NS_TOUCH_EVENT,
+    MOZ_ASSERT(mClass == NS_TOUCH_EVENT,
                "Duplicate() must be overridden by sub class");
     // Not copying widget, it is a weak reference.
     WidgetTouchEvent* result = new WidgetTouchEvent(false, message, nullptr);
diff --git a/widget/gtk/nsGtkKeyUtils.cpp b/widget/gtk/nsGtkKeyUtils.cpp
index ac3b23b8a73..4414358b882 100644
--- a/widget/gtk/nsGtkKeyUtils.cpp
+++ b/widget/gtk/nsGtkKeyUtils.cpp
@@ -646,7 +646,7 @@ KeymapWrapper::InitInputEvent(WidgetInputEvent& aInputEvent,
          GetBoolName(aInputEvent.modifiers & MODIFIER_NUMLOCK),
          GetBoolName(aInputEvent.modifiers & MODIFIER_SCROLLLOCK)));
 
-    switch(aInputEvent.eventStructType) {
+    switch(aInputEvent.mClass) {
         case NS_MOUSE_EVENT:
         case NS_MOUSE_SCROLL_EVENT:
         case NS_WHEEL_EVENT:
diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
index 5ec28e6d565..7ec743acec0 100644
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -6211,7 +6211,7 @@ NS_IMETHODIMP
 nsWindow::BeginMoveDrag(WidgetMouseEvent* aEvent)
 {
     NS_ABORT_IF_FALSE(aEvent, "must have event");
-    NS_ABORT_IF_FALSE(aEvent->eventStructType == NS_MOUSE_EVENT,
+    NS_ABORT_IF_FALSE(aEvent->mClass == NS_MOUSE_EVENT,
                       "event must have correct struct type");
 
     GdkWindow *gdk_window;
@@ -6234,7 +6234,7 @@ nsWindow::BeginResizeDrag(WidgetGUIEvent* aEvent,
 {
     NS_ENSURE_ARG_POINTER(aEvent);
 
-    if (aEvent->eventStructType != NS_MOUSE_EVENT) {
+    if (aEvent->mClass != NS_MOUSE_EVENT) {
         // you can only begin a resize drag with a mouse event
         return NS_ERROR_INVALID_ARG;
     }
diff --git a/widget/nsGUIEventIPC.h b/widget/nsGUIEventIPC.h
index 9b226c80799..e558e8736fa 100644
--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -45,7 +45,8 @@ struct ParamTraits<mozilla::WidgetEvent>
 
   static void Write(Message* aMsg, const paramType& aParam)
   {
-    WriteParam(aMsg, (uint8_t) aParam.eventStructType);
+    WriteParam(aMsg,
+      static_cast<mozilla::EventClassIDType>(aParam.mClass));
     WriteParam(aMsg, aParam.message);
     WriteParam(aMsg, aParam.refPoint);
     WriteParam(aMsg, aParam.time);
@@ -55,14 +56,14 @@ struct ParamTraits<mozilla::WidgetEvent>
 
   static bool Read(const Message* aMsg, void** aIter, paramType* aResult)
   {
-    uint8_t eventStructType = 0;
-    bool ret = ReadParam(aMsg, aIter, &eventStructType) &&
+    mozilla::EventClassIDType eventClassID = 0;
+    bool ret = ReadParam(aMsg, aIter, &eventClassID) &&
                ReadParam(aMsg, aIter, &aResult->message) &&
                ReadParam(aMsg, aIter, &aResult->refPoint) &&
                ReadParam(aMsg, aIter, &aResult->time) &&
                ReadParam(aMsg, aIter, &aResult->timeStamp) &&
                ReadParam(aMsg, aIter, &aResult->mFlags);
-    aResult->eventStructType = static_cast<nsEventStructType>(eventStructType);
+    aResult->mClass = static_cast<mozilla::EventClassID>(eventClassID);
     return ret;
   }
 };
diff --git a/widget/shared/WidgetEventImpl.cpp b/widget/shared/WidgetEventImpl.cpp
index f22224dbc69..06278461823 100644
--- a/widget/shared/WidgetEventImpl.cpp
+++ b/widget/shared/WidgetEventImpl.cpp
@@ -46,25 +46,25 @@ WidgetEvent::As##aName() const \
 bool
 WidgetEvent::IsQueryContentEvent() const
 {
-  return eventStructType == NS_QUERY_CONTENT_EVENT;
+  return mClass == NS_QUERY_CONTENT_EVENT;
 }
 
 bool
 WidgetEvent::IsSelectionEvent() const
 {
-  return eventStructType == NS_SELECTION_EVENT;
+  return mClass == NS_SELECTION_EVENT;
 }
 
 bool
 WidgetEvent::IsContentCommandEvent() const
 {
-  return eventStructType == NS_CONTENT_COMMAND_EVENT;
+  return mClass == NS_CONTENT_COMMAND_EVENT;
 }
 
 bool
 WidgetEvent::IsNativeEventDelivererForPlugin() const
 {
-  return eventStructType == NS_PLUGIN_EVENT;
+  return mClass == NS_PLUGIN_EVENT;
 }
 
 
@@ -215,7 +215,7 @@ WidgetEvent::IsTargetedAtFocusedContent() const
 bool
 WidgetEvent::IsAllowedToDispatchDOMEvent() const
 {
-  switch (eventStructType) {
+  switch (mClass) {
     case NS_MOUSE_EVENT:
     case NS_POINTER_EVENT:
       // We want synthesized mouse moves to cause mouseover and mouseout
diff --git a/widget/windows/KeyboardLayout.cpp b/widget/windows/KeyboardLayout.cpp
index 9a86006d43a..db2f6812e6c 100644
--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -280,7 +280,7 @@ ModifierKeyState::InitInputEvent(WidgetInputEvent& aInputEvent) const
 {
   aInputEvent.modifiers = mModifiers;
 
-  switch(aInputEvent.eventStructType) {
+  switch(aInputEvent.mClass) {
     case NS_MOUSE_EVENT:
     case NS_MOUSE_SCROLL_EVENT:
     case NS_WHEEL_EVENT:
@@ -296,10 +296,10 @@ ModifierKeyState::InitInputEvent(WidgetInputEvent& aInputEvent) const
 void
 ModifierKeyState::InitMouseEvent(WidgetInputEvent& aMouseEvent) const
 {
-  NS_ASSERTION(aMouseEvent.eventStructType == NS_MOUSE_EVENT ||
-               aMouseEvent.eventStructType == NS_WHEEL_EVENT ||
-               aMouseEvent.eventStructType == NS_DRAG_EVENT ||
-               aMouseEvent.eventStructType == NS_SIMPLE_GESTURE_EVENT,
+  NS_ASSERTION(aMouseEvent.mClass == NS_MOUSE_EVENT ||
+               aMouseEvent.mClass == NS_WHEEL_EVENT ||
+               aMouseEvent.mClass == NS_DRAG_EVENT ||
+               aMouseEvent.mClass == NS_SIMPLE_GESTURE_EVENT,
                "called with non-mouse event");
 
   if (XRE_GetWindowsEnvironment() == WindowsEnvironmentType_Metro) {
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index ccb678bd65a..6eaccb305b6 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -1538,7 +1538,7 @@ nsWindow::BeginResizeDrag(WidgetGUIEvent* aEvent,
 {
   NS_ENSURE_ARG_POINTER(aEvent);
 
-  if (aEvent->eventStructType != NS_MOUSE_EVENT) {
+  if (aEvent->mClass != NS_MOUSE_EVENT) {
     // you can only begin a resize drag with a mouse event
     return NS_ERROR_INVALID_ARG;
   }
diff --git a/widget/windows/winrt/MetroWidget.cpp b/widget/windows/winrt/MetroWidget.cpp
index 30c387d73c9..243f0c6c681 100644
--- a/widget/windows/winrt/MetroWidget.cpp
+++ b/widget/windows/winrt/MetroWidget.cpp
@@ -651,7 +651,7 @@ bool
 MetroWidget::DispatchScrollEvent(mozilla::WidgetGUIEvent* aEvent)
 {
   WidgetGUIEvent* newEvent = nullptr;
-  switch(aEvent->eventStructType) {
+  switch(aEvent->mClass) {
     case NS_WHEEL_EVENT:
     {
       WidgetWheelEvent* oldEvent = aEvent->AsWheelEvent();
diff --git a/widget/xpwidgets/PuppetWidget.cpp b/widget/xpwidgets/PuppetWidget.cpp
index 3928abe49e1..baf23bce12f 100644
--- a/widget/xpwidgets/PuppetWidget.cpp
+++ b/widget/xpwidgets/PuppetWidget.cpp
@@ -290,7 +290,7 @@ PuppetWidget::DispatchEvent(WidgetGUIEvent* event, nsEventStatus& aStatus)
     mIMEComposing = true;
   }
   uint32_t seqno = kLatestSeqno;
-  switch (event->eventStructType) {
+  switch (event->mClass) {
   case NS_COMPOSITION_EVENT:
     seqno = event->AsCompositionEvent()->mSeqno;
     break;