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.
Click the Refresh Firefox button in the upper-right corner of the Troubleshooting Information page.
To continue, click Refresh Firefox in the confirmation window that opens.
Firefox will close to refresh itself. When finished, a window will list your imported information. Click Finish and Firefox will open.
Note: There's also a Refresh Firefox button in the Firefox Safe Mode window, if you can't start Firefox normally. If you are unable to find the Refresh Firefox button on your version of Firefox, you can also do a manual refresh by creating a new profile. See instructions on how to create a new profile.