Download a new copy of the Firefox program and save the DMG file to the desktop * Firefox 4.0.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 [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder], so you won't lose your bookmarks and other personal data.