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.
We actually did some testing along those lines, and it came back pretty negative. When "Mozilla" was too prominent, people did not expect it to work with other browsers, and wouldn't even attempt to use Persona.