Depends on the computer.
On my main desktop, I adore Firefox. I used to detest it in favour of Opera back in the day, but Opera was... unstable. The main browsers at the time were IE and Firefox. I loved Opera's clean interface. Nobody loves the actual page pushed down to a small square because of Google Toolbar, which I needed for searches. Opera heftily provided me with a miniature Google Search bar. So I lived with Opera. Then, randomly, I decided to try out Firefox because there was this one session that Opera pissed me off with the incompatibilities. And then, I was greeted by a UI similar to Opera. A stable browser+the opera UI= Browser of my dreams. I quit using Opera altogether. Eventually, all the other browsers adopted the idea, but Firefox grew on me, I suppose.
On the netbook, I use Firefox Aurora and Chrome. Painfully said, Aurora (which I am using now) takes an awful lot of time to load with Intel Atom. I use Firefox as an alternative when I want to browse something rapidly, but for an upcoming intense session of browsing, I like Aurora. I am not using mainstream Firefox only because I like the name Aurora. :P It's a very good name, better than Firefox. Yeah, I know, not much of a reason, but meh. 2lazy2install regular Firefox.