--- a/media/webrtc/signaling/src/mediapipeline/MediaPipeline.h
+++ b/media/webrtc/signaling/src/mediapipeline/MediaPipeline.h
@@ -446,16 +446,23 @@ public:
 #if !defined(MOZILLA_EXTERNAL_LINKAGE)
         , last_img_(-1)
 #endif // MOZILLA_INTERNAL_API
     {
     }
 
     ~PipelineListener()
     {
+      if (NS_IsMainThread()) {
+        // This would happen if the cycle collector is destructing us, in which
+        // case it'd be too late to dispatch something to main thread anyway.
+        conduit_ = nullptr;
+        return;
+      }
+
       // release conduit on mainthread.  Must use forget()!
       nsresult rv = NS_DispatchToMainThread(new
         ConduitDeleteEvent(conduit_.forget()));
       MOZ_ASSERT(!NS_FAILED(rv),"Could not dispatch conduit shutdown to main");
       if (NS_FAILED(rv)) {
         MOZ_CRASH();
       }
     }
@@ -608,16 +615,23 @@ class MediaPipelineReceiveAudio : public
   class PipelineListener : public GenericReceiveListener {
    public:
     PipelineListener(SourceMediaStream * source, TrackID track_id,
                      const RefPtr<MediaSessionConduit>& conduit,
                      bool queue_track);
 
     ~PipelineListener()
     {
+      if (NS_IsMainThread()) {
+        // This would happen if the cycle collector is destructing us, in which
+        // case it'd be too late to dispatch something to main thread anyway.
+        conduit_ = nullptr;
+        return;
+      }
+
       // release conduit on mainthread.  Must use forget()!
       nsresult rv = NS_DispatchToMainThread(new
         ConduitDeleteEvent(conduit_.forget()));
       MOZ_ASSERT(!NS_FAILED(rv),"Could not dispatch conduit shutdown to main");
       if (NS_FAILED(rv)) {
         MOZ_CRASH();
       }
     }