I use Firefox .9.3 (too lazy to upgrade) and have really enjoyed it. My favorite feature is a simple one -- that little search bar to the right of the url bar. I use Yahoo as my homepage because I like to read the news when I open it up, but now searching Google is more easy than ever
As for extensions, the only one I use is the Web Developer one. It's so nice to have all of those options right there for easy access. Makes troubleshooting the few websites I've done since I've run Firefox go much quicker as I can find the source of the problem.
All in all, I'm extremely satisfied with Mozilla Firefox. It's a great web browser no matter who you are =P