Very cool add-on. The only features I used are media playback and downloads integration. However, I have never had Firefox crash on me in ages (years?), but after installing this add-on and attempting to watch a youtube video, Firefox crashed! (It did work for a few videos before that.) Since removing the add-on Firefox hasn't crashed with youtube video playback, so I'm 99.99999% certain that the add-on, while very cool, definitely jeopardizes stability. What is weird is that the tab process should have died given e10s ("multiprocess"/multithreading), but instead the whole browser did. I'll try again some time in the future. Oh, and it doesn't work with Spotify for whatever reason (DRM?).