I've had some real problems with Firefox on both MacOS10.4 and Windows XP. The mac version just keeps locking up and I get a beach ball of death. The windows version just disappears randomly. I've had been running the beta for a few months and dutifully uploading all the crash dumps hoping this would help the developers. When Firefox finally went gold I upgraded assuming I would get some stability back. I didn't. After a week or so I was on the edge of giving up. I decided to see if removing an extension would help. I have stumbleupon and Google Toolbar installed, both of which are up to date.
Well to cut a long story short I removed the Google toolbar and I haven't had a crash on either platform for a few days now. Hope that helps.
Moral of the story: Software is complex so cut it to the bone before putting your fist through your monitor 
Thank you. Great tip. I got so tired of Firefox crashing my entire system.
Firefox has been disappointing me lately… seems like it doesn’t like my system at all. But, great tip nonetheless!
OK great I want to remove Google toolbar but I don’t know how – can you please help. I hate Google!! Thanks
@Rich: to remove the toolbar:
Goto tools menu and select addons, the select “Google Toolbar for Firefox” and click disable or uninstall. You then need to restart FireFox.
I have to say though that Firefox seems to be a lot more stable and Google seem to have fixed any problems I was getting. I’ve reinstalled Google Toolbar recently and not experienced any crashes.