Mozilla is shutting down Persona later this year (November 2016). See:
https://mail.mozilla.org/pipermail/persona-notices/2016/000005.html
We currently support Persona as an authentication provider, and should communicate its upcoming shutdown to users in advance of service termination.
We built this support originally in 2013 when @indygreg (who works at Mozilla) requested it in T3958. Mozilla did not adopt Phabricator and presumably never used it themselves, and no users on this install other than he or I have ever mentioned Persona since we built the integration, as far as I can tell.
Given that we have no direct knowledge of users and Mozilla's own rationale for abandoning the project is "low, declining usage", it is plausible that no installs ever used this integration, so I think we can be relatively cavalier about removing it.