Hi I'm the Mozilla intern that developed this and would really love some feedback! It's super-experimental right now, but feel free to try it out! I plan to release a more formal post later once some more bugs are squashed.
My slightly less positive feedback would be, it seems dodgy that it uses personatest.org instead of persona.org (and there's no website at www.personatest.org verifying its legitimacy). Kind of makes me anxious that I'm being phished. Any reason for this? You surely don't want people getting used to entering their Persona credentials at sites other than persona.org.
Otherwise it seems great :)