--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -195,22 +195,16 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(CustomElementRegistry)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mCustomDefinitions)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mWhenDefinedPromiseMap)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mWindow)
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(CustomElementRegistry)
-  for (auto iter = tmp->mCustomDefinitions.Iter(); !iter.Done(); iter.Next()) {
-    aCallbacks.Trace(&iter.UserData()->mPrototype,
-                     "mCustomDefinitions prototype",
-                     aClosure);
-  }
-
   for (ConstructorMap::Enum iter(tmp->mConstructors); !iter.empty(); iter.popFront()) {
     aCallbacks.Trace(&iter.front().mutableKey(),
                      "mConstructors key",
                      aClosure);
   }
   NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
 NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
@@ -587,51 +581,49 @@ CustomElementRegistry::Define(const nsAS
    * 8. If this CustomElementRegistry's element definition is running flag is set,
    *    then throw a "NotSupportedError" DOMException and abort these steps.
    */
   if (mIsCustomDefinitionRunning) {
     aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
     return;
   }
 
-  JS::Rooted<JSObject*> constructorPrototype(cx);
+  JS::Rooted<JS::Value> constructorPrototype(cx);
   nsAutoPtr<LifecycleCallbacks> callbacksHolder(new LifecycleCallbacks());
   nsTArray<RefPtr<nsAtom>> observedAttributes;
   { // Set mIsCustomDefinitionRunning.
     /**
      * 9. Set this CustomElementRegistry's element definition is running flag.
      */
     AutoSetRunningFlag as(this);
 
     { // Enter constructor's compartment.
       /**
        * 10.1. Let prototype be Get(constructor, "prototype"). Rethrow any exceptions.
        */
       JSAutoCompartment ac(cx, constructor);
-      JS::Rooted<JS::Value> prototypev(cx);
       // The .prototype on the constructor passed could be an "expando" of a
       // wrapper. So we should get it from wrapper instead of the underlying
       // object.
-      if (!JS_GetProperty(cx, constructor, "prototype", &prototypev)) {
+      if (!JS_GetProperty(cx, constructor, "prototype", &constructorPrototype)) {
         aRv.StealExceptionFromJSContext(cx);
         return;
       }
 
       /**
        * 10.2. If Type(prototype) is not Object, then throw a TypeError exception.
        */
-      if (!prototypev.isObject()) {
+      if (!constructorPrototype.isObject()) {
         aRv.ThrowTypeError<MSG_NOT_OBJECT>(NS_LITERAL_STRING("constructor.prototype"));
         return;
       }
-
-      constructorPrototype = &prototypev.toObject();
     } // Leave constructor's compartment.
 
-    JS::Rooted<JSObject*> constructorProtoUnwrapped(cx, js::CheckedUnwrap(constructorPrototype));
+    JS::Rooted<JSObject*> constructorProtoUnwrapped(
+      cx, js::CheckedUnwrap(&constructorPrototype.toObject()));
     if (!constructorProtoUnwrapped) {
       // If the caller's compartment does not have permission to access the
       // unwrapped prototype then throw.
       aRv.Throw(NS_ERROR_DOM_SECURITY_ERR);
       return;
     }
 
     { // Enter constructorProtoUnwrapped's compartment
@@ -748,19 +740,17 @@ CustomElementRegistry::Define(const nsAS
     return;
   }
 
   RefPtr<CustomElementDefinition> definition =
     new CustomElementDefinition(nameAtom,
                                 localNameAtom,
                                 &aFunctionConstructor,
                                 Move(observedAttributes),
-                                constructorPrototype,
-                                callbacks,
-                                0 /* TODO dependent on HTML imports. Bug 877072 */);
+                                callbacks);
 
   CustomElementDefinition* def = definition.get();
   mCustomDefinitions.Put(nameAtom, definition.forget());
 
   MOZ_ASSERT(mCustomDefinitions.Count() == mConstructors.count(),
              "Number of entries should be the same");
 
   /**
@@ -1074,17 +1064,16 @@ CustomElementReactionsStack::InvokeReact
 
 //-----------------------------------------------------
 // CustomElementDefinition
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(CustomElementDefinition)
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(CustomElementDefinition)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mConstructor)
-  tmp->mPrototype = nullptr;
   tmp->mCallbacks = nullptr;
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(CustomElementDefinition)
   mozilla::dom::LifecycleCallbacks* callbacks = tmp->mCallbacks.get();
 
   if (callbacks->mAttributeChangedCallback.WasPassed()) {
     NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb,
@@ -1107,37 +1096,32 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(
     cb.NoteXPCOMChild(callbacks->mAdoptedCallback.Value());
   }
 
   NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mConstructor");
   cb.NoteXPCOMChild(tmp->mConstructor);
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(CustomElementDefinition)
-  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mPrototype)
 NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
 NS_IMPL_CYCLE_COLLECTION_ROOT_NATIVE(CustomElementDefinition, AddRef)
 NS_IMPL_CYCLE_COLLECTION_UNROOT_NATIVE(CustomElementDefinition, Release)
 
 
 CustomElementDefinition::CustomElementDefinition(nsAtom* aType,
                                                  nsAtom* aLocalName,
                                                  Function* aConstructor,
                                                  nsTArray<RefPtr<nsAtom>>&& aObservedAttributes,
-                                                 JS::Handle<JSObject*> aPrototype,
-                                                 LifecycleCallbacks* aCallbacks,
-                                                 uint32_t aDocOrder)
+                                                 LifecycleCallbacks* aCallbacks)
   : mType(aType),
     mLocalName(aLocalName),
     mConstructor(new CustomElementConstructor(aConstructor)),
     mObservedAttributes(Move(aObservedAttributes)),
-    mPrototype(aPrototype),
-    mCallbacks(aCallbacks),
-    mDocOrder(aDocOrder)
+    mCallbacks(aCallbacks)
 {
 }
 
 
 //-----------------------------------------------------
 // CustomElementUpgradeReaction
 
 /* virtual */ void

--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -145,45 +145,37 @@ struct CustomElementDefinition
 {
   NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(CustomElementDefinition)
   NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(CustomElementDefinition)
 
   CustomElementDefinition(nsAtom* aType,
                           nsAtom* aLocalName,
                           Function* aConstructor,
                           nsTArray<RefPtr<nsAtom>>&& aObservedAttributes,
-                          JS::Handle<JSObject*> aPrototype,
-                          mozilla::dom::LifecycleCallbacks* aCallbacks,
-                          uint32_t aDocOrder);
+                          mozilla::dom::LifecycleCallbacks* aCallbacks);
 
   // The type (name) for this custom element, for <button is="x-foo"> or <x-foo>
   // this would be x-foo.
   RefPtr<nsAtom> mType;
 
   // The localname to (e.g. <button is=type> -- this would be button).
   RefPtr<nsAtom> mLocalName;
 
   // The custom element constructor.
   RefPtr<CustomElementConstructor> mConstructor;
 
   // The list of attributes that this custom element observes.
   nsTArray<RefPtr<nsAtom>> mObservedAttributes;
 
-  // The prototype to use for new custom elements of this type.
-  JS::Heap<JSObject *> mPrototype;
-
   // The lifecycle callbacks to call for this custom element.
   UniquePtr<mozilla::dom::LifecycleCallbacks> mCallbacks;
 
   // A construction stack. Use nullptr to represent an "already constructed marker".
   nsTArray<RefPtr<Element>> mConstructionStack;
 
-  // The document custom element order.
-  uint32_t mDocOrder;
-
   bool IsCustomBuiltIn()
   {
     return mType != mLocalName;
   }
 
   bool IsInObservedAttributeList(nsAtom* aName)
   {
     if (mObservedAttributes.IsEmpty()) {