Thanks for the enlightenment. Did you not consider going for adoption by regular Mozilla users, and only then try expanding to the wide open internet? I have the feeling that by trying to tackle everyone at once you ran into brand recognition problems.

For reference, I pulled this login screen from an image search on google: http://raygun.io/blog/2013/08/mozilla-persona-support/

The question must be asked: Does Persona have any chance of being used, in that context?

This branding issue can be compensated by having the browser pull out a dedicated login interface, or by really leveraging the known Mozilla brand (accepting the negative aspect you presented, which is inescapable). Without compensation, it is indeed a lost cause.