Thats the thing, it is broke in the eyes of Mozilla, and the larger web community. It's now 2 years old, while the implementation of HTML 5 continues to progress. It also stopped seeing any security updates at the end of January.

Why HTML 5 couldn't just be deployed as one solid update to the spec is beyond me though. It is frustrating to watch the W3C, and the browser manufacturers continue to make this transition a messy one.

I'm not a daily Firefox user, so I can't comment on what UI changes occurred between 3.6 and the current 12 version. Mozilla does claim the add ons you have should have upgrades or just work in the new version, they've been putting effort into making them more compatible with a wider range of versions of the browser.