The code nsRefPtr<nsSupportsWeakReference> tmp2 = do_QueryObject(tmp); which appears in PeerConnectionImpl.cpp is an abuse of do_QueryObject, since nsSupportsWeakReference does not have an IID, so what happens is that tmp gets reinterpreted as an nsSupportsWeakReference*. :bz suggested that the quick fix is to use static_cast<PeerConnectionObserver>(tmp.get()) instead. An alternative would be to change the code generation to add an IID to PeerConnectionObserver so that you could write nsRefPtr<PeerConnectionObserver> pco = do_QueryReferent(mPCObserver); directly and avoid the kludge that is do_QueryObjectReferent. Had it been possible to use an XPIDL interface in the first place then nsCOMPtr<nsIPeerConnectionObserver> pco = do_QueryReferent(mPCObserver); would also have sufficed.