--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -3768,17 +3768,21 @@ class HTMLMediaElement::ShutdownObserver
     Unsubscribed
   };
 
 public:
   NS_DECL_ISUPPORTS
 
   NS_IMETHOD Observe(nsISupports*, const char* aTopic, const char16_t*) override
   {
-    MOZ_DIAGNOSTIC_ASSERT(mPhase == Phase::Subscribed);
+    if (mPhase != Phase::Subscribed) {
+      // Bail out if we are not subscribed for this might be called even after
+      // |nsContentUtils::UnregisterShutdownObserver(this)|.
+      return NS_OK;
+    }
     MOZ_DIAGNOSTIC_ASSERT(mWeak);
     if (strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0) {
       mWeak->NotifyShutdownEvent();
     }
     return NS_OK;
   }
   void Subscribe(HTMLMediaElement* aPtr)
   {