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)
Nevertheless, I'm hopeful that we'll integrate Persona into the Firefox Accounts workflow, but that's still only part of the problem that Firefox Accounts is trying to solve. :)
(Why wasn't it there at first? We were still working out protocol / data format details, and sticking with a known username/password system reduced the number of variables while reinventing Sync.)
Firefox Accounts, in addition to Persona, could actually move the needle. If Mozilla would drop the idea that email providers should be IdP and instead step into the role themselves (like they kinda are with Accounts) this could all work.
More simply put why not do Persona as an SSO solution independent of email providers (Mozilla or a partner is the single IdP). Optionally attach data to that identity (several technical ways to do this) to hold more data such as that envisioned by Firefox Accounts (email, TOS acceptance, ...).
Also, this is a new occurrence of scrypt used in the wild.
I wanna love Mozilla (even bought a Firefox OS phone!) but this is pretty low-level.