Plus, Mozilla is dog-fooding Persona all over the place[1], so we're personally invested in getting this thing right and keeping it working.
[1]: Bugzilla, MDN, Etherpad, Mozillians, Metrics, Popcorn, OpenBadges, Marketplace, Add-on Builder, Flicks, and Affiliates all use Persona, with more to come.
BrowserID is new, and awesome. My fear is that a good universal login needs to support both Mobile Safari and Google Chrome on Android, and due to their childish fighting, it's unlikely anything client side can work on both, to the detriment of users and Internet security as a whole.
Desktop browsers are a lot easier (extensions, and browser choice), but mobile/tablets makes this all a lot harder than it was a few years ago. :(