--- a/dom/base/nsDOMMutationObserver.h
+++ b/dom/base/nsDOMMutationObserver.h
@@ -21,16 +21,17 @@
 #include "nsClassHashtable.h"
 #include "nsNodeUtils.h"
 #include "nsIDOMMutationEvent.h"
 #include "nsWrapperCache.h"
 #include "mozilla/dom/MutationObserverBinding.h"
 #include "nsIDocument.h"
 #include "mozilla/dom/Animation.h"
 #include "nsIAnimationObserver.h"
+#include "nsGlobalWindow.h"
 
 class nsDOMMutationObserver;
 using mozilla::dom::MutationObservingInfo;
 
 class nsDOMMutationRecord final : public nsISupports,
                                   public nsWrapperCache
 {
   virtual ~nsDOMMutationRecord() {}
@@ -604,21 +605,17 @@ protected:
   void ScheduleForRun();
   void RescheduleForRun();
 
   nsDOMMutationRecord* CurrentRecord(nsIAtom* aType);
   bool HasCurrentRecord(const nsAString& aType);
 
   bool Suppressed()
   {
-    if (mOwner) {
-      nsCOMPtr<nsIDocument> d = mOwner->GetExtantDoc();
-      return d && d->IsInSyncOperation();
-    }
-    return false;
+    return mOwner && nsGlobalWindow::Cast(mOwner)->IsInSyncOperation();
   }
 
   static void HandleMutationsInternal();
 
   static void AddCurrentlyHandlingObserver(nsDOMMutationObserver* aObserver,
                                            uint32_t aMutationLevel);
 
   nsCOMPtr<nsPIDOMWindowInner>                       mOwner;

--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -1308,16 +1308,21 @@ public:
 
   already_AddRefed<nsWindowRoot> GetWindowRootOuter();
   already_AddRefed<nsWindowRoot> GetWindowRoot(mozilla::ErrorResult& aError);
 
   mozilla::dom::Performance* GetPerformance();
 
   void UpdateTopInnerWindow();
 
+  virtual bool IsInSyncOperation() override
+  {
+    return GetExtantDoc() && GetExtantDoc()->IsInSyncOperation();
+  }
+
 protected:
   // Web IDL helpers
 
   // Redefine the property called aPropName on this window object to be a value
   // property with the value aValue, much like we would do for a [Replaceable]
   // property in IDL.
   void RedefineProperty(JSContext* aCx, const char* aPropName,
                         JS::Handle<JS::Value> aValue,

--- a/dom/base/nsIGlobalObject.h
+++ b/dom/base/nsIGlobalObject.h
@@ -69,16 +69,18 @@ public:
 
   void UnregisterHostObjectURI(const nsACString& aURI);
 
   // Any CC class inheriting nsIGlobalObject should call these 2 methods if it
   // exposes the URL API.
   void UnlinkHostObjectURIs();
   void TraverseHostObjectURIs(nsCycleCollectionTraversalCallback &aCb);
 
+  virtual bool IsInSyncOperation() { return false; }
+
 protected:
   virtual ~nsIGlobalObject();
 
   void
   StartDying()
   {
     mIsDying = true;
   }