http://people.mozilla.com/%7Efaaborg/files/projects/firefoxA...
There's also some work on integrating existing e-mail providers, so you can get instant identities:
http://identity.mozilla.com/post/17207734786/id-provider-sup...
This is probably the web-thing I'm most excited about today, even though the development is kinda slow. I really want to implement it as a sole identity provider on my websites some day.
EDIT: I also began writing a Safari extension that would work like the mockup above, but gave up halfway since it was too convoluted and totally insecure... maybe I should try writing a SIMBL plugin or something like that.