Quite easy way @Noman_Ali please follow the below steps respectively.
- To reset entire Firefox is open your Firefox browser.
- Navigate to help menu on upper right hand.
- Now choose troubleshooting information option fro the list.
- On that page, click on refresh/reset Firefox button.
- A pop up will be appear and ask about to go proceed and also tell you that all addons and settings will be remove after Firefox refresh. Then finally click on Refresh Firefox button.
Firefox will restart automatically, after restarting Firefox now it has been reset on its default condition. That' all.
Hopefully, this answered your question.