Try hitting '''F11'''.

Make sure that you do not run Firefox in Full Screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F). *https://support.mozilla.org/kb/how-to-use-full-screen When you are in Full Screen mode, hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear. You can use one of these to leave Full Screen mode: *click the Maximize button (top right corner of the Navigation Toolbar) to leave Full Screen mode *click the Full Screen button in the "3-bar" menu button drop-down list *right-click empty space on a toolbar and choose "Exit Full Screen Mode" *press the F11 key Try to rename (or delete) the xulstore.json file in the Firefox profile folder. You can use this button to go to the current Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- It is possible that there is a problem with files in the profile folder that store the extensions registry. Delete the extensions.* files (e.g. extensions.json, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder to reset the extensions registry. *https://support.mozilla.org/kb/Profiles New files will be created when required. See "Corrupt extension files": *http://kb.mozillazine.org/Unable_to_install_themes_or_extensions *https://support.mozilla.org/kb/Unable+to+install+add-ons If you see disabled or not compatible extensions in "Firefox menu button/Tools > Add-ons > Extensions" then click the Tools button at the left end of the Search bar to check if there is a compatibility update available. If this didn't help then delete addons.json and possibly prefs.js in case there is a problem with preferences.