As I understand it, the authentication token you get back from BrowserID is signed by the entity that verified that the email address is real; at the moment, on

As I understand it, the authentication token you get back from BrowserID is signed by the entity that verified that the email address is real; at the moment, only browserid.org is signing tokens, but presumably there'll be some scheme where you say "I have a token for foo@example.com, so I'll request the public key of example.com and validate the signature on this token". I guess it's possible for somebody to set up a website that will sign absolutely any token anybody offers them, but (a) as a website operator, you can tell that the token "foo@example.com signed by example.com" is different from "foo@example.com signed by evil.com" and (b) you could always have a blacklist/whitelist of entities you trust to verify BrowserIDs.