Reset Firefox settings to default

From https://support.mozilla.org/en-US/kb/reset-preferences-fix-problems

1. Open Firefox
2. Click the menu button  and then click help  .
3. From the Help menu choose Troubleshooting Information.

   If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.

4. Click the Refresh Firefox button in the upper-right corner of the Troubleshooting Information page.

   

5. To continue, click Refresh Firefox in the confirmation window that opens.
6. Firefox will close to refresh itself. When finished, a window will list your imported information. Click Finish and Firefox will open.