If there are problems with updating or with the permissions then best 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 application and save the disk image file to the desktop *Firefox 39.0: https://www.mozilla.org/en-US/firefox/all/ *Trash the current Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean (re)install *Install the new version that you have downloaded *https://support.mozilla.org/kb/Installing+Firefox+on+Mac Your personal data is stored elsewhere in the Firefox profile folder, so you won't lose your bookmarks and other personal data when you uninstall and (re)install or update Firefox. *http://kb.mozillazine.org/Profile_folder_-_Firefox