I hear a lot of people extolling the virtues of Opera, and I have a copy installed on my machine, but I haven't really noticed that much of a speed difference between it and FF. Maybe the Linux versions are less different speed-wise than the Windows versions?
The main reason I use Firefox is because of all the developer extensions (HTML tidy, web developer toolbar, color picker, color contrast tester, etc), and I don't know how many of these are available on Opera.
Now my phone is a different story. I started using Dolphin a while back, which was a decent browser, but I've since switched to Chrome. I just prefer the way it works and renders pages.