diff --git a/dom/media/FileBlockCache.cpp b/dom/media/FileBlockCache.cpp
index 6461b6fc5bea..4cc9ef219c19 100644
--- a/dom/media/FileBlockCache.cpp
+++ b/dom/media/FileBlockCache.cpp
@@ -74,9 +74,16 @@ void FileBlockCache::Close()
     // opening more streams, while the media cache is shutting down and
     // releasing memory etc! Also note we close mFD in the destructor so
     // as to not disturb any IO that's currently running.
-    nsCOMPtr<nsIRunnable> event = new ShutdownThreadEvent(mThread);
-    mThread = nullptr;
-    NS_DispatchToMainThread(event);
+    nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
+    if (mainThread) {
+      nsCOMPtr<nsIRunnable> event = new ShutdownThreadEvent(mThread);
+      mainThread->Dispatch(event.forget(), NS_DISPATCH_NORMAL);
+    } else {
+      // we're on Mainthread already, *and* the event queues are already
+      // shut down, so no events should occur - certainly not creations of
+      // new streams.
+      mThread->Shutdown();
+    }
   }
 }
 
diff --git a/dom/media/MediaCache.cpp b/dom/media/MediaCache.cpp
index ab5248faf687..aed2a6695d13 100644
--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -1409,8 +1409,14 @@ MediaCache::QueueUpdate()
   if (mUpdateQueued)
     return;
   mUpdateQueued = true;
-  nsCOMPtr<nsIRunnable> event = new UpdateEvent();
-  NS_DispatchToMainThread(event);
+  // XXX MediaCache does updates when decoders are still running at
+  // shutdown and get freed in the final cycle-collector cleanup.  So
+  // don't leak a runnable in that case.
+  nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
+  if (mainThread) {
+    nsCOMPtr<nsIRunnable> event = new UpdateEvent();
+    mainThread->Dispatch(event.forget(), NS_DISPATCH_NORMAL);
+  }
 }
 
 void