I wish they would implement an (open, as in, non-corporate) social network and single-sign-on system on top of that. It would be a killer combination, making a firefox tablet more easily marketable.
As for Single-Sign-On, Persona can already take care of that, today, for virtually any domain / browser combo. You can help out on that, too: https://github.com/mozilla/browserid
The future will be even better if the Tent and Diaspora folks make it easy for their servers to also function as Identity Providers for Persona's protocol. FWIW, it seems like there's interest on both sides to make this happen.
(edited to remove slangy word "rumblings")