The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.
To Reset Firefox do the following:
Go to Firefox > Help > Troubleshooting Information.
Click the "Reset Firefox" button.
Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
Firefox will open with all factory defaults applied.