While awaiting answers here, I’d advise you to also try the Firefox Support Forum which is specialized on problems with this browser.
http://forums.mozillazine.org/viewforum.php?f=38
As regards my suggestions, you stand a good chance of success if you backup your bookmarks and re-install Firefox. The less add-ons, extensions and toolbars you install with it, the better as regards stability, security and speed. So use the minimum required and get them from the Firefox site itself, as they can be malicious. And install them one at a time, waiting to see if any trouble appears.
If you don’t want to re-install, try disabling all said add-ons, extensions and toolbars to see if it solves the problem. If it does, re-enable them one at a time to find the culprit.
Furthermore, if Firefox is frozen for some reason, then it should be closed from the Task Manager, otherwise it doesn’t close properly and on restarting it will give the message you mention.
Last but not least, the Task Manager is not a good indicator of memory usage, as it shows what’s reserved for it, not what is actually used by it. To reduce this figure, minimize and re-maximize Firefox.