Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F). If the menu bar is hidden then press the F10 key or hold down the Alt key to make the menu bar appear. Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible: "View > Toolbars" ---- If Firefox doesn't start or doesn't display web pages then a possible cause is security software (firewall) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program. Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process. See: *https://support.mozilla.org/kb/Server+not+found *https://support.mozilla.org/kb/Firewalls