I regularly ran Firefox 3.6 (I'm on v4 now) for weeks at a time, and never had such issues. I'd guess it's an extension or plugin you had installed.
My biggest problem with Firefox (v4 only) is shutting it down takes ages as it iterates through every file in the cache, which is encrypted on my machine. I limited my cache size to 50MB, which seems to have limited the pause times to something reasonable.
I cannot stand Chrome though; I hate the way it selects text on the page with the mouse (I'm a compulsive selection-reader), and the cobbled-together extension replacements for the Bookmarks menu don't work nearly as well as a proper menu.
I do have a few extensions installed & enabled -- AdBlock Plus, Firebug, Greasemonkey (rarely used, I should just uninstall it), Flashblock, and YSlow.
But, I've seen similar behavior on at least one other system, and the only extension we had in common there was AdBlock Plus. The behavior also predates Greasemonkey.
The other system is newer, and the slowdown there is less pronounced (though still noticeable), which leads me to suspect that there's some task that runs regularly that performs well enough on fast enough systems to be less problematic, but becomes a miserable dog on older systems -- a pretty typical case of programmer "fast enough for me".
Apropos of nothing: I don't get you compulsive selection readers. I'm always a little bemused when one of you speaks up. Why do you do that? It's so weird. :-)
My biggest problem with Firefox (v4 only) is shutting it down takes ages as it iterates through every file in the cache, which is encrypted on my machine. I limited my cache size to 50MB, which seems to have limited the pause times to something reasonable.
I cannot stand Chrome though; I hate the way it selects text on the page with the mouse (I'm a compulsive selection-reader), and the cobbled-together extension replacements for the Bookmarks menu don't work nearly as well as a proper menu.