Mozilla begins testing Account Manager for Firefox to simplify web logins
To that end, Mozilla has put together a draft spec (which you can view at Google Docs) which would add new extensions to the HTTP client/server communication process for authentication. They've also released an experimental add-on for Firefox called Account Manager which you can use to demo proposed system on a handful of sites. Right now, Facebook, Google, Yahoo! and three Mozilla sites (Addons, Personas, and Bugzilla) should work -- Mozilla has hard-coded support for them into the addon itself.
Facebook worked nicely. After logging in, the key icon in my toolbar illuminated and clicking it revealed that, yes, I was logged in to Facebook as myself. I didn't have the same luck with Google, but that's to be expected at this point.
Anyone who wants to code in support for the spec on his can check out the draft doc and implement support on their sites and the code for the Account Manager addon is available as well.