Stan James' Outfoxed is a Firefox extension to support rating pages and distributing the metadata across social networks. James built Outfoxed in less than six months, as part of a Masters' thesis, and the site has a detailed narration of the system's design, implementation, and consequences.
I'm not a huge fan of explicit rating systems, but Outfoxed is a good case study of how to build new functionality directly into an extensible browser.
Hey, isn't about time for someone to start a new commercial effort around Firefox to make it "safe for business"? Do some heavy duty engineering to make the browser much more efficient (memory consumption is ridiculous), provide some cleaned up well maintained groupware extensions, tie the whole bundle into readily available backend systems (Exchange, wiki & weblog tools, del.icio.us clone), and call it a low end Groove. Might be worth somebody's effort.