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 42.0: 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 like bookmarks is stored in the Firefox profile folder, so you won't lose personal data when you uninstall and (re)install or update Firefox, but make sure NOT to remove personal data when you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data.