I have been a long timer Firefox user, since the really early days, but lately the memory bloat has just increased beyond what is comfortable. My browser is sometimes running for weeks at a time and it is not unnatural to see Firefox reach the 800MB mark even though I might only have 2 tabs open. The consequence of this is that the rest of my programs, xemacs mail iChat and Serendipity Blackmagic become unresponsive for a few seconds as I switch over to them. The worst part of the experience though is that Firefox will sometimes lock up all the tabs while it is trying to connect to some ad site or doing garbage collection.
I switched to Opera a week ago, and after a small transition period I am no really happy with it, It is always responsive, does not use much memory and has this most awesome feature called Speed dial.