How to reset Firefox

How do I reset Firefox to its default state?

1. At the top of the Firefox window, click the Help menu and select Troubleshooting Information.

   

2. Click the Reset Firefox… button in the upper-right corner of the Troubleshooting Information page.

   

3. To continue, click Reset Firefox in the confirmation window that opens.
4. Firefox will close and be reset. When it's done, a window will list the information that was imported. Click Finish and Firefox will open.

#from : https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems