Best bet is probably to download the full installer, close Firefox, and run the installer. -> http://www.mozilla.org/en-US/firefox/new/

If there are 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 disk image (dmg) file to the desktop * Firefox 7.0.x: http://www.mozilla.com/en-US/firefox/all.html * Trash the current Firefox application to do a clean (re-)install * Install the new version that you have downloaded Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data. * http://kb.mozillazine.org/Profile_folder_-_Firefox