Master$hake

This is happening because firefox is trying to access a profile that is still in use by your system either hung or you have a hidden process accessing your profile like AV. I usually keep a back up of my profile in case I want to reload firefox for this reason or an upgrade. http://www.mozilla.org/support/firefox/profile#locate Grab your profile uninstall and then reinstall and load your profile if the other advice here doesn't work