after religiously supporting firefox throughout it's early years, i recently switched to chrome because since v3 firefox has gotten slower and slower. 10 seconds or more to load, memory leaks and several seconds to shut down. i still use it for dev, because of the addons, but use chrome for everyday browsing and most testing.
however update 3.6.10 has really speeded up the browser on my machine (vista). 4 seconds or less to load and generally feels much more responsive. it's a joy a use again. tabs load quickly, menus load quickly.
i have the following addons installed: lastpass, xmarks, webdev, colorzilla, firebug. but i don't think the previous speed problems were down to these, because at one time or another i've disabled/uninstalled all in the quest for speed.
anyway, if you've gone off firefox as i did it's worth giving it another go after this latest update.