Some background about the relationship between Persona and Firefox Accounts:

https://wiki.mozilla.org/Identity/Firefox_Accounts

https://groups.google.com/d/msg/mozilla.mozillians/pv5VN2dNY...

tl;dr: it's coming, and this is an important engineering milestone on that path. I'm sure Lloyd and callahad will be chiming in with more details.

(Source: Mozilla employee, not on the Identity team)