--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1014,19 +1014,18 @@ CustomElementReactionsStack::Enqueue(Ele
   mBackupQueue.AppendElement(aElement);
   elementData->mReactionQueue.AppendElement(aReaction);
 
   if (mIsBackupQueueProcessing) {
     return;
   }
 
   CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
-  RefPtr<ProcessBackupQueueRunnable> processBackupQueueRunnable =
-    new ProcessBackupQueueRunnable(this);
-  context->DispatchToMicroTask(processBackupQueueRunnable.forget());
+  RefPtr<BackupQueueMicroTask> bqmt = new BackupQueueMicroTask(this);
+  context->DispatchMicroTaskRunnable(bqmt.forget());
 }
 
 void
 CustomElementReactionsStack::InvokeBackupQueue()
 {
   // Check backup queue size in order to reduce function call overhead.
   if (!mBackupQueue.IsEmpty()) {
     // Upgrade reactions won't be scheduled in backup queue and the exception of

--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -308,34 +308,32 @@ private:
    * Invoke custom element reactions
    * https://html.spec.whatwg.org/multipage/scripting.html#invoke-custom-element-reactions
    */
   void InvokeReactions(ElementQueue* aElementQueue, nsIGlobalObject* aGlobal);
 
   void Enqueue(Element* aElement, CustomElementReaction* aReaction);
 
 private:
-  class ProcessBackupQueueRunnable : public mozilla::Runnable {
+  class BackupQueueMicroTask final : public mozilla::MicroTaskRunnable {
     public:
-      explicit ProcessBackupQueueRunnable(
+      explicit BackupQueueMicroTask(
         CustomElementReactionsStack* aReactionStack)
-        : Runnable(
-            "dom::CustomElementReactionsStack::ProcessBackupQueueRunnable")
+        : MicroTaskRunnable()
         , mReactionStack(aReactionStack)
       {
         MOZ_ASSERT(!mReactionStack->mIsBackupQueueProcessing,
                    "mIsBackupQueueProcessing should be initially false");
         mReactionStack->mIsBackupQueueProcessing = true;
       }
 
-      NS_IMETHOD Run() override
+      virtual void Run(AutoSlowOperation& aAso) override
       {
         mReactionStack->InvokeBackupQueue();
         mReactionStack->mIsBackupQueueProcessing = false;
-        return NS_OK;
       }
 
     private:
       RefPtr<CustomElementReactionsStack> mReactionStack;
   };
 };
 
 class CustomElementRegistry final : public nsISupports,