[TheCoelacanth]

Persona is the protocol that the sites use to communicate with the identity provider. If they support Persona they will support Persona authentication from any email provider that chooses to support Persona and from the fallback provider that Mozilla provides.

[unhammer]

Well, since they get the email address, they can easily check that it ends in @gmail and stop everyone else. Of course, only supporting gmail means they have to write _more code_ than supporting every provider, so lets hope lazyness wins.