--- a/ipc/mscom/WeakRef.h
+++ b/ipc/mscom/WeakRef.h
@@ -105,17 +105,17 @@ protected:
   void Lock();
   void Unlock();
 
   typedef BaseAutoLock<WeakReferenceSupport> AutoLock;
   friend class BaseAutoLock<WeakReferenceSupport>;
 
 private:
   RefPtr<detail::SharedRef> mSharedRef;
-  ULONG                     mRefCnt;
+  Atomic<ULONG>             mRefCnt;
   Flags                     mFlags;
   CRITICAL_SECTION          mCSForQI;
 };
 
 class WeakRef final : public IWeakReference
 {
 public:
   // IUnknown