Right now it's impossible to write certain tests from javascript because imgIRequest doesn't store a strong reference to its listener, so the imgIDecoderObserver that xpconnect creates out of thin air can go away at inopportune times, and with no warning. We should either make imgRequestProxy::mListener a strong reference and cycle collect imgIDecoderObserver implementations and imgRequestProxys, or we should make imgRequestProxy::mListener an nsWeakPtr and deal with the fact that we can't depend on our listener existing for all time.

Jeff and I talked about this a while back, but I don't remember what we settled on.