Try <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:

• 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

• https://support.mozilla.org/kb/how-to-use-full-screen

If that doesn't help then it is possible that there is a problem with the xulstore.json file.

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