I had been using Firefox since like 0.7 or something. About a month ago though, I switched to Opera 9, which was still in beta at the time.
I really like Opera and it even has Bittorrent built in as of version 9. Firefox just seemed to really hog memory (its a feature, not a bug they say) and if I left the browser open for any length of time it would get pretty laggy. That is not acceptable for me, especially on the hardware I am running on.
Give Opera a try if you haven't, there are some things I liked better with Firefox but overall I'm very pleased with Opera.