Hi, you may be in Full Screen Mode: Point to the top of the screen, then right click > Exit Full Screen Mode. You can also press the F11 key, but depending on the configuration of your computer, you may also need to hold down Fn at the same time. More - Use Firefox in Full Screen mode.

If that doesn't help, hold down Shift when you start Firefox to start in Safe Mode. More - Use Troubleshoot Mode in Firefox

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Try to rename/remove the xulstore.json file in the Firefox profile folder if the above isn't working.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox