Persona didn't have the notion of a global password -- the ideal flow had you authenticating with your email provider, on their domain, which was often completely transparent in cases where you were already signed in to that provider.

Try logging into 123done.org with whatever you want @mockmyid.com to get a sense of what that would ideally look like. My talk from PyCon 2013 goes into a little detail about how the protocol works: https://www.youtube.com/watch?v=nJff23UdNAI