What else do you need to know?
If you have problems with updating or with the permissions then easiest is to download the full version and trash the currently installed version to do a clean install of the new version.
Download a new copy of the Firefox program and save the DMG file to the desktop
Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data.
I have this problem too and doing what was suggested - downloading from scratch does not solve the problem. Still get the same error message. Just tried that.
Did you move the Firefox 3.0.6 installation to the trash?