Ever since 2.2, firefox hasn't worked properly on either of my computers using it, and with every update since 3, it has only gotten slower and more prone to crashes. It's not an addon conflict, I've checked. I'm also not the only one with such problems. Hence why I always refer to firefox as failfox. The only thing keeping it on my computers is adblock and noscript - otherwise, I'd have opera for sure. Opera has a million more features, loads faster, has a better interface, and has a lot of hidden goodies that I happen to like (try breaking a tab out into a new window in firefox without having to reload it).

You should see firefox 2.19 on my old computer - less hardware, but it's fast as opera, with noscript, adblock plus, tabMix plus and speed dial added on.