--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -707,18 +707,17 @@ nsDOMEvent::DuplicatePrivateData()
         new WidgetSimpleGestureEvent(false, msg, nullptr, 0, 0.0);
       simpleGestureEvent->
         AssignSimpleGestureEventData(*oldSimpleGestureEvent, true);
       newEvent = simpleGestureEvent;
       break;
     }
     case NS_TRANSITION_EVENT:
     {
-      InternalTransitionEvent* oldTransitionEvent =
-        static_cast<InternalTransitionEvent*>(mEvent);
+      InternalTransitionEvent* oldTransitionEvent = mEvent->AsTransitionEvent();
       InternalTransitionEvent* transitionEvent =
          new InternalTransitionEvent(false, msg,
                                      oldTransitionEvent->propertyName,
                                      oldTransitionEvent->elapsedTime,
                                      oldTransitionEvent->pseudoElement);
       transitionEvent->AssignTransitionEventData(*oldTransitionEvent, true);
       newEvent = transitionEvent;
       break;

--- a/content/events/src/nsDOMTransitionEvent.cpp
+++ b/content/events/src/nsDOMTransitionEvent.cpp
@@ -41,42 +41,49 @@ nsDOMTransitionEvent::Constructor(const 
                                   mozilla::ErrorResult& aRv)
 {
   nsCOMPtr<mozilla::dom::EventTarget> t = do_QueryInterface(aGlobal.GetAsSupports());
   nsRefPtr<nsDOMTransitionEvent> e = new nsDOMTransitionEvent(t, nullptr, nullptr);
   bool trusted = e->Init(t);
 
   aRv = e->InitEvent(aType, aParam.mBubbles, aParam.mCancelable);
 
-  e->TransitionEvent()->propertyName = aParam.mPropertyName;
-  e->TransitionEvent()->elapsedTime = aParam.mElapsedTime;
-  e->TransitionEvent()->pseudoElement = aParam.mPseudoElement;
+  InternalTransitionEvent* internalEvent = e->mEvent->AsTransitionEvent();
+  internalEvent->propertyName = aParam.mPropertyName;
+  internalEvent->elapsedTime = aParam.mElapsedTime;
+  internalEvent->pseudoElement = aParam.mPseudoElement;
 
   e->SetTrusted(trusted);
   return e.forget();
 }
 
 NS_IMETHODIMP
 nsDOMTransitionEvent::GetPropertyName(nsAString & aPropertyName)
 {
-  aPropertyName = TransitionEvent()->propertyName;
+  aPropertyName = mEvent->AsTransitionEvent()->propertyName;
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMTransitionEvent::GetElapsedTime(float *aElapsedTime)
 {
   *aElapsedTime = ElapsedTime();
   return NS_OK;
 }
 
+float
+nsDOMTransitionEvent::ElapsedTime()
+{
+  return mEvent->AsTransitionEvent()->elapsedTime;
+}
+
 NS_IMETHODIMP
 nsDOMTransitionEvent::GetPseudoElement(nsAString& aPseudoElement)
 {
-  aPseudoElement = TransitionEvent()->pseudoElement;
+  aPseudoElement = mEvent->AsTransitionEvent()->pseudoElement;
   return NS_OK;
 }
 
 nsresult
 NS_NewDOMTransitionEvent(nsIDOMEvent **aInstancePtrResult,
                          mozilla::dom::EventTarget* aOwner,
                          nsPresContext *aPresContext,
                          InternalTransitionEvent* aEvent)

--- a/content/events/src/nsDOMTransitionEvent.h
+++ b/content/events/src/nsDOMTransitionEvent.h
@@ -2,17 +2,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #ifndef nsDOMTransitionEvent_h_
 #define nsDOMTransitionEvent_h_
 
 #include "nsDOMEvent.h"
 #include "nsIDOMTransitionEvent.h"
-#include "mozilla/ContentEvents.h"
+#include "mozilla/EventForwards.h"
 #include "mozilla/dom/TransitionEventBinding.h"
 
 class nsAString;
 
 class nsDOMTransitionEvent : public nsDOMEvent,
                              public nsIDOMTransitionEvent
 {
 public:
@@ -35,22 +35,12 @@ public:
   {
     return mozilla::dom::TransitionEventBinding::Wrap(aCx, aScope, this);
   }
 
   // xpidl implementation
   // GetPropertyName(nsAString& aPropertyName)
   // GetPseudoElement(nsAString& aPreudoElement)
 
-  float ElapsedTime()
-  {
-    return TransitionEvent()->elapsedTime;
-  }
-
-private:
-  mozilla::InternalTransitionEvent* TransitionEvent() {
-    NS_ABORT_IF_FALSE(mEvent->eventStructType == NS_TRANSITION_EVENT,
-                      "unexpected struct type");
-    return static_cast<mozilla::InternalTransitionEvent*>(mEvent);
-  }
+  float ElapsedTime();
 };
 
 #endif /* !defined(nsDOMTransitionEvent_h_) */

--- a/content/events/src/nsEventDispatcher.cpp
+++ b/content/events/src/nsEventDispatcher.cpp
@@ -741,17 +741,17 @@ nsEventDispatcher::CreateEvent(mozilla::
     case NS_SIMPLE_GESTURE_EVENT:
       return NS_NewDOMSimpleGestureEvent(aDOMEvent, aOwner, aPresContext,
                static_cast<WidgetSimpleGestureEvent*>(aEvent));
     case NS_TOUCH_EVENT:
       return NS_NewDOMTouchEvent(aDOMEvent, aOwner, aPresContext,
                                  static_cast<WidgetTouchEvent*>(aEvent));
     case NS_TRANSITION_EVENT:
       return NS_NewDOMTransitionEvent(aDOMEvent, aOwner, aPresContext,
-               static_cast<InternalTransitionEvent*>(aEvent));
+                                      aEvent->AsTransitionEvent());
     case NS_ANIMATION_EVENT:
       return NS_NewDOMAnimationEvent(aDOMEvent, aOwner, aPresContext,
                static_cast<InternalAnimationEvent*>(aEvent));
     default:
       // For all other types of events, create a vanilla event object.
       return NS_NewDOMEvent(aDOMEvent, aOwner, aPresContext, aEvent);
     }
   }