I notice that the firefox 3 is still have some bugs. So here is the simple to this solution right now, "WAIT UNTIL THE FIREFOX HES BEEN OFFICIALLY RELEASE" . Why do you have to use the beta version? So mean while, please use the old Firefox 2.0. This step below will install you the previous version of firefox:
sudo apt-get install firefox-2If somehow you still wanted to be a tester user to help firefox to improve and report the bugs in firefox 3, you can always use it by selecting at the "Application > Internet > Firefox 3" menu. So it's your choice.