Reset Firefox to its default state
- Click the menu button and then click help .
- From the
- If you're unable to access themenu, type about:support in your address bar to bring up the Troubleshooting Information page.
menu choose .
- Click the
- To continue, click in the confirmation window that opens.
- Firefox will close and be reset. When it's done, a window will list the information that was imported. Click and Firefox will open.
Note: There's also a Reset Firefox button in the Firefox Safe Mode window, if you can't start Firefox normally.
FROM https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings