I'm still playing around with Firefox. And it's taken a while because the issue is so random and infrequent. But I became suspicious that the freeze ups would occur when the browser was Syncing (integrated sync feature).
I uninstalled the browser, removed the old profile (and Mozilla directories). Reinstalled with a fresh profile. And enabled my Sync again. Now interestingly I don't think I'm freezing up anymore, but now I'm getting a sync error message once in a while (kinda random and infrequent like my freeze ups were).
So I did some searching on this. And it turns out there are some sync issues on going.
http://twitter.com/#!/mozservices
"We know there are still Sync nodes having issues. We are working diligently to correct. Sorry for the errors (if you're affected)."
And if you are having Sync issues with Firefox, they setup a website to help them debug the issue.
http://blog.mozilla.com/jv/2011/10/03/help-mozilla-services-operations-debug-whats-wrong-with-sync/
I just sent them my log file.
edit: nice timing, not 10 minutes after posting this, Firefox freezes up again. I jinxed it. I'm also starting to notice I usually have a Flash video playing when these issues happen. Hmm...