--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -646,17 +646,17 @@ nsDOMEvent::DuplicatePrivateData()
       InternalMutationEvent* oldMutationEvent =
         static_cast<InternalMutationEvent*>(mEvent);
       mutationEvent->AssignMutationEventData(*oldMutationEvent, true);
       newEvent = mutationEvent;
       break;
     }
     case NS_FORM_EVENT:
     {
-      InternalFormEvent* oldFormEvent = static_cast<InternalFormEvent*>(mEvent);
+      InternalFormEvent* oldFormEvent = mEvent->AsFormEvent();
       InternalFormEvent* formEvent = new InternalFormEvent(false, msg);
       formEvent->AssignFormEventData(*oldFormEvent, true);
       newEvent = formEvent;
       break;
     }
     case NS_FOCUS_EVENT:
     {
       InternalFocusEvent* newFocusEvent = new InternalFocusEvent(false, msg);

--- a/content/html/content/src/HTMLFormElement.cpp
+++ b/content/html/content/src/HTMLFormElement.cpp
@@ -681,26 +681,24 @@ nsresult
 HTMLFormElement::BuildSubmission(nsFormSubmission** aFormSubmission, 
                                  WidgetEvent* aEvent)
 {
   NS_ASSERTION(!mPendingSubmission, "tried to build two submissions!");
 
   // Get the originating frame (failure is non-fatal)
   nsGenericHTMLElement* originatingElement = nullptr;
   if (aEvent) {
-    if (NS_FORM_EVENT == aEvent->eventStructType) {
-      nsIContent* originator =
-        static_cast<InternalFormEvent*>(aEvent)->originator;
+    InternalFormEvent* formEvent = aEvent->AsFormEvent();
+    if (formEvent) {
+      nsIContent* originator = formEvent->originator;
       if (originator) {
         if (!originator->IsHTML()) {
           return NS_ERROR_UNEXPECTED;
         }
-        originatingElement =
-          static_cast<nsGenericHTMLElement*>(
-            static_cast<InternalFormEvent*>(aEvent)->originator);
+        originatingElement = static_cast<nsGenericHTMLElement*>(originator);
       }
     }
   }
 
   nsresult rv;
 
   //
   // Get the submission object