I use Firefox almost since the beginning, but it became very bloated and seems to have major issues regarding memory management. Sometimes I can close all tabs and memory usage is still over 500mb and response times from clicks etc. become very huge. Only a restart helps in such a case (but that takes a while since Firefox needs like a minute to close…).
I’ve also used Opera 10 from time to time, but I felt some features/extensions are missing (mainly dev tools and adblocker) AND the diff-view on Google Code was messed up, but it’s obviously fixed since 10.63. Now I tried version 11 Beta and it’s just awesome! It’s possible now to install/uninstall “real” extensions without restart. Overal performance is very good and (on the long run) much better than Firefox. Only cons for me are the lack of some good extension, but I guess they will come with time. Also the beta still has some small bugs, but they will probably dissapear in the final version.
What you think of Opera? You can try the beta here.