Persona is an awesome way to verify email address ownership, but that's only part of what Sync and the Firefox Marketplace need. For instance, Sync needs a user-memorable password for client-side encryption, and Marketplace needs the ability to force users to re-authenticate before a purchase, which is only feasible in a centralized system. Then there are COPPA concerns: We don't want to build an age gate into Persona, but we need one on Sync and Marketplace. Firefox Accounts lets us do that once, up front, and be on our merry way.

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.)