--- a/content/base/public/nsINode.h
+++ b/content/base/public/nsINode.h
@@ -1488,17 +1488,20 @@ protected:
 
   void ClearSubtreeRootPointer()
   {
     mSubtreeRoot = nullptr;
   }
 
 public:
   // Optimized way to get classinfo.
-  virtual nsXPCClassInfo* GetClassInfo() = 0;
+  virtual nsXPCClassInfo* GetClassInfo()
+  {
+    return nullptr;
+  }
 
   // Makes nsINode object to keep aObject alive.
   void BindObject(nsISupports* aObject);
   // After calling UnbindObject nsINode object doesn't keep
   // aObject alive anymore.
   void UnbindObject(nsISupports* aObject);
 
   /**

--- a/js/xpconnect/src/XPCQuickStubs.cpp
+++ b/js/xpconnect/src/XPCQuickStubs.cpp
@@ -1008,9 +1008,17 @@ xpc_qsAssertContextOK(JSContext *cx)
 {
     XPCJSContextStack* stack = XPCJSRuntime::Get()->GetJSContextStack();
 
     JSContext *topJSContext = stack->Peek();
 
     // This is what we're actually trying to assert here.
     NS_ASSERTION(cx == topJSContext, "wrong context on XPCJSContextStack!");
 }
+
+void
+xpcObjectHelper::AssertGetClassInfoResult()
+{
+    MOZ_ASSERT(mXPCClassInfo ||
+               static_cast<nsINode*>(GetCanonical())->IsDOMBinding(),
+               "GetClassInfo() should only return null for new DOM bindings!");
+}
 #endif

--- a/js/xpconnect/src/xpcObjectHelper.h
+++ b/js/xpconnect/src/xpcObjectHelper.h
@@ -69,20 +69,25 @@ public:
           return mXPCClassInfo;
         if (!mClassInfo)
             mClassInfo = do_QueryInterface(mObject);
         return mClassInfo;
     }
     nsXPCClassInfo *GetXPCClassInfo()
     {
         if (!mXPCClassInfo) {
-            if (mIsNode)
-                mXPCClassInfo = static_cast<nsINode*>(GetCanonical())->GetClassInfo();
-            else
+            if (mIsNode) {
+                mXPCClassInfo =
+                    static_cast<nsINode*>(GetCanonical())->GetClassInfo();
+#ifdef DEBUG
+                AssertGetClassInfoResult();
+#endif
+            } else {
                 CallQueryInterface(mObject, getter_AddRefs(mXPCClassInfo));
+            }
         }
         return mXPCClassInfo;
     }
 
     already_AddRefed<nsXPCClassInfo> forgetXPCClassInfo()
     {
         GetXPCClassInfo();
 
@@ -124,16 +129,20 @@ protected:
     }
 
     nsCOMPtr<nsISupports>    mCanonicalStrong;
     nsISupports*             mCanonical;
 
 private:
     xpcObjectHelper(xpcObjectHelper& aOther) MOZ_DELETE;
 
+#ifdef DEBUG
+    void AssertGetClassInfoResult();
+#endif
+
     nsISupports*             mObject;
     nsWrapperCache*          mCache;
     nsCOMPtr<nsIClassInfo>   mClassInfo;
     nsRefPtr<nsXPCClassInfo> mXPCClassInfo;
     bool                     mIsNode;
 };
 
 #endif