diff --git a/content/base/src/FragmentOrElement.cpp b/content/base/src/FragmentOrElement.cpp
index 0dac4e4a2e82..e3627d51a7ef 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->mClass != NS_MUTATION_EVENT ||
+                 aVisitor.mEvent->mClass != eMutationEventClass ||
                  aVisitor.mDOMEvent,
                  "Mutation event dispatched in native anonymous content!?!");
 #endif
diff --git a/content/html/content/src/HTMLInputElement.cpp b/content/html/content/src/HTMLInputElement.cpp
index ce45fcfc4dac..08922e04d98c 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->mClass == NS_MUTATION_EVENT) {
+      aVisitor.mEvent->mClass == eMutationEventClass) {
     return false;
   }
 
diff --git a/dom/events/Event.cpp b/dom/events/Event.cpp
index f212c608beda..6f8ee130cc1c 100644
--- a/dom/events/Event.cpp
+++ b/dom/events/Event.cpp
@@ -174,7 +174,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Event)
       case eClipboardEventClass:
         tmp->mEvent->AsClipboardEvent()->clipboardData = nullptr;
         break;
-      case NS_MUTATION_EVENT:
+      case eMutationEventClass:
         tmp->mEvent->AsMutationEvent()->mRelatedNode = nullptr;
         break;
       case eFocusEventClass:
@@ -216,7 +216,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(Event)
         NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mEvent->clipboardData");
         cb.NoteXPCOMChild(tmp->mEvent->AsClipboardEvent()->clipboardData);
         break;
-      case NS_MUTATION_EVENT:
+      case eMutationEventClass:
         NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mEvent->mRelatedNode");
         cb.NoteXPCOMChild(tmp->mEvent->AsMutationEvent()->mRelatedNode);
         break;
diff --git a/dom/events/EventDispatcher.cpp b/dom/events/EventDispatcher.cpp
index af59ce752c4b..753991467a11 100644
--- a/dom/events/EventDispatcher.cpp
+++ b/dom/events/EventDispatcher.cpp
@@ -685,7 +685,7 @@ EventDispatcher::CreateEvent(EventTarget* aOwner,
 
   if (aEvent) {
     switch(aEvent->mClass) {
-    case NS_MUTATION_EVENT:
+    case eMutationEventClass:
       return NS_NewDOMMutationEvent(aDOMEvent, aOwner, aPresContext,
                                     aEvent->AsMutationEvent());
     case eGUIEventClass:
diff --git a/dom/events/EventNameList.h b/dom/events/EventNameList.h
index 39ef04628518..2e00e3501ac5 100644
--- a/dom/events/EventNameList.h
+++ b/dom/events/EventNameList.h
@@ -577,31 +577,31 @@ NON_IDL_EVENT(MozMouseHittest,
 NON_IDL_EVENT(DOMAttrModified,
               NS_MUTATION_ATTRMODIFIED,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 NON_IDL_EVENT(DOMCharacterDataModified,
               NS_MUTATION_CHARACTERDATAMODIFIED,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 NON_IDL_EVENT(DOMNodeInserted,
               NS_MUTATION_NODEINSERTED,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 NON_IDL_EVENT(DOMNodeRemoved,
               NS_MUTATION_NODEREMOVED,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 NON_IDL_EVENT(DOMNodeInsertedIntoDocument,
               NS_MUTATION_NODEINSERTEDINTODOCUMENT,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 NON_IDL_EVENT(DOMNodeRemovedFromDocument,
               NS_MUTATION_NODEREMOVEDFROMDOCUMENT,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 NON_IDL_EVENT(DOMSubtreeModified,
               NS_MUTATION_SUBTREEMODIFIED,
               EventNameType_HTMLXUL,
-              NS_MUTATION_EVENT)
+              eMutationEventClass)
 
 NON_IDL_EVENT(DOMActivate,
               NS_UI_ACTIVATE,
diff --git a/dom/events/InternalMutationEvent.h b/dom/events/InternalMutationEvent.h
index 58f0b2815eb6..f78368e6ad64 100644
--- a/dom/events/InternalMutationEvent.h
+++ b/dom/events/InternalMutationEvent.h
@@ -18,16 +18,16 @@ class InternalMutationEvent : public WidgetEvent
 public:
   virtual InternalMutationEvent* AsMutationEvent() MOZ_OVERRIDE { return this; }
 
-  InternalMutationEvent(bool aIsTrusted, uint32_t aMessage) :
-    WidgetEvent(aIsTrusted, aMessage, NS_MUTATION_EVENT),
-    mAttrChange(0)
+  InternalMutationEvent(bool aIsTrusted, uint32_t aMessage)
+    : WidgetEvent(aIsTrusted, aMessage, eMutationEventClass)
+    , mAttrChange(0)
   {
     mFlags.mCancelable = false;
   }
 
   virtual WidgetEvent* Duplicate() const MOZ_OVERRIDE
   {
-    MOZ_ASSERT(mClass == NS_MUTATION_EVENT,
+    MOZ_ASSERT(mClass == eMutationEventClass,
                "Duplicate() must be overridden by sub class");
     InternalMutationEvent* result = new InternalMutationEvent(false, message);
     result->AssignMutationEventData(*this, true);
diff --git a/widget/BasicEvents.h b/widget/BasicEvents.h
index 5d0ef4675ab5..2ece97dffc34 100644
--- a/widget/BasicEvents.h
+++ b/widget/BasicEvents.h
@@ -68,7 +68,7 @@ enum EventClassID MOZ_ENUM_TYPE(EventClassIDType)
   ePluginEventClass,                 // WidgetPluginEvent
 
   // InternalMutationEvent.h (dom/events)
-  NS_MUTATION_EVENT                  // InternalMutationEvent
+  eMutationEventClass                // InternalMutationEvent
 };
 
 } // namespace mozilla