Hit '''F11'''.

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: *press the F11 key *click the Maximize button at the right end of the Tab bar *click the Full Screen button in the "3-bar" menu button drop-down list *right-click empty space on a toolbar and click "Exit Full Screen Mode" *https://support.mozilla.org/kb/how-to-use-full-screen ----- 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