--- a/content/events/src/nsDOMCommandEvent.cpp
+++ b/content/events/src/nsDOMCommandEvent.cpp
@@ -28,17 +28,17 @@ NS_INTERFACE_MAP_BEGIN(nsDOMCommandEvent
 NS_INTERFACE_MAP_END_INHERITING(nsDOMEvent)
 
 NS_IMPL_ADDREF_INHERITED(nsDOMCommandEvent, nsDOMEvent)
 NS_IMPL_RELEASE_INHERITED(nsDOMCommandEvent, nsDOMEvent)
 
 NS_IMETHODIMP
 nsDOMCommandEvent::GetCommand(nsAString& aCommand)
 {
-  nsIAtom* command = static_cast<WidgetCommandEvent*>(mEvent)->command;
+  nsIAtom* command = mEvent->AsCommandEvent()->command;
   if (command) {
     command->ToString(aCommand);
   } else {
     aCommand.Truncate();
   }
   return NS_OK;
 }
 
@@ -46,17 +46,17 @@ NS_IMETHODIMP
 nsDOMCommandEvent::InitCommandEvent(const nsAString& aTypeArg,
                                     bool aCanBubbleArg,
                                     bool aCancelableArg,
                                     const nsAString& aCommand)
 {
   nsresult rv = nsDOMEvent::InitEvent(aTypeArg, aCanBubbleArg, aCancelableArg);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  static_cast<WidgetCommandEvent*>(mEvent)->command = do_GetAtom(aCommand);
+  mEvent->AsCommandEvent()->command = do_GetAtom(aCommand);
   return NS_OK;
 }
 
 nsresult NS_NewDOMCommandEvent(nsIDOMEvent** aInstancePtrResult,
                                mozilla::dom::EventTarget* aOwner,
                                nsPresContext* aPresContext,
                                WidgetCommandEvent* aEvent)
 {

--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -655,18 +655,17 @@ nsDOMEvent::DuplicatePrivateData()
       InternalFocusEvent* newFocusEvent = new InternalFocusEvent(false, msg);
       InternalFocusEvent* oldFocusEvent = mEvent->AsFocusEvent();
       newFocusEvent->AssignFocusEventData(*oldFocusEvent, true);
       newEvent = newFocusEvent;
       break;
     }
     case NS_COMMAND_EVENT:
     {
-      WidgetCommandEvent* oldCommandEvent =
-        static_cast<WidgetCommandEvent*>(mEvent);
+      WidgetCommandEvent* oldCommandEvent = mEvent->AsCommandEvent();
       WidgetCommandEvent* commandEvent =
         new WidgetCommandEvent(false, mEvent->userType,
                                oldCommandEvent->command, nullptr);
       commandEvent->AssignCommandEventData(*oldCommandEvent, true);
       newEvent = commandEvent;
       break;
     }
     case NS_UI_EVENT:

--- a/content/events/src/nsEventDispatcher.cpp
+++ b/content/events/src/nsEventDispatcher.cpp
@@ -732,17 +732,17 @@ nsEventDispatcher::CreateEvent(mozilla::
     case NS_SVGZOOM_EVENT:
       return NS_NewDOMSVGZoomEvent(aDOMEvent, aOwner, aPresContext,
                                    static_cast<WidgetGUIEvent*>(aEvent));
     case NS_SMIL_TIME_EVENT:
       return NS_NewDOMTimeEvent(aDOMEvent, aOwner, aPresContext, aEvent);
 
     case NS_COMMAND_EVENT:
       return NS_NewDOMCommandEvent(aDOMEvent, aOwner, aPresContext,
-                                   static_cast<WidgetCommandEvent*>(aEvent));
+                                   aEvent->AsCommandEvent());
     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,