The only bad thing with this new Gmail integration is that it gets more annoying if you have multiple Gmail accounts, or aren't logged in to the Gmail account you want to use.
And some things, like using myname+website@gmail.com doesn't work at all.
And some things, like using myname+website@gmail.com doesn't work at all.
EDIT: Now I remember my use case.
My email is actually myname+randomstring+website@otherdomain.com, and so I want to sign in with the same randomstring every time. Previously when signing in to Persona with my Gmail email address it would suggest all alternatives, and I'd choose the appropriate one. Now it logs in with Gmail directly instead.
Of course, this is such an edge case that it's probably not worth doing anything about. And I can simply sign into Persona with myname@otherdomain.com for the previous functionality.
Run it with the following command-line arguments: -ProfileManager -new-instance
That brings up a profile picker/creater. Each profile is isolated, different plugins, different cookies, etc. The only information leakage is when a plugin keeps its own data outside of the profile directory, like Adobe Flash keeps its version of cookies in a per-user directory rather than a per-profile directory.