Persona is going to be decommed November 30th, 2016.
It is highly unlikely that anyone is currently using persona as a real
login method at this point.
See T10125 for some additional discussion.
In theory, this should also regenerate the actual login sprites with scripts/celerity/generate_sprites.php, although that's kind of a pain (you need optipng installed) and doesn't really matter.
(There's also probably little real value in spriting these anyway, and replacing them with a dynamic CSS file after T11217 provides that capability is probably a good step in the long run.)
Since at least one install does actually use this (Fedora QA) and it costs us very little to keep it around for 3-4 more months until it actually breaks, I'm inclined to just wait until Mozilla terminates it on Nov 30th. I'll accept this upstream after that.
The Persona login dialog still sort of works I guess but now has a prominent notice that it's no longer supported, so I think we're good to get rid of it. I've disabled the provider on this install.