It's not really a "password manager", it uses signed assertions instead of plaintext passwords. It is similar to client certificates, but with a much better UI.
Also, in the future you probably won't even need to contact Mozilla anywhere in the process: your browser will store your certificates for you, natively.
Plus, your keys are stored in the browser, according to callahad, so Mozilla doesn't have to know where you signed up: http://news.ycombinator.com/item?id=4232774
Also, in the future you probably won't even need to contact Mozilla anywhere in the process: your browser will store your certificates for you, natively.