Ignore last.If i remember right I tried that and had a hard job reinstalling it as it says Firefox is still running. I believe I had to reinstall it with no internet connection or in safe mode. I must have downloaded it on to my computer without running it. I am sure someone will come up with an easier remedy.
Firefox 3.6 is more stable! However the cause of this is the cache becomes bloated and firefox is still running as it has put around 1GB into the cache which in turn puts it into the ram, hence the slow down and lack of ability to close down properly. You need to clear the cache and then upgrade to 3.6