You can check for problems with preferences. Delete possible user.js and numbered prefs-##.js files and rename/remove the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases. *http://kb.mozillazine.org/Preferences_not_saved *http://kb.mozillazine.org/Resetting_preferences You can use this button to go to the current Firefox profile folder: *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- Try to download the full version of the current release. Trash the currently installed Firefox application to do a clean reinstall. Verify that all files in Firefox.app are gone. */Applications/Firefox.app/ */Applications/Firefox.app/updates/ Download a new copy of the Firefox application and save the Disk Image file to the desktop *Firefox 50.1.0: *https://www.mozilla.org/en-US/firefox/all/ *Trash the currently installed Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean reinstall *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 reinstall 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.