- Open Firefox browser.
- Go to Tools and select Options. Firefox Options window will appear.
- Press OK button in the bottom of the Options window.
- Open Firefox web browser.
- Go to Tools in menu bar and select Options link. Now Firefox Options window will appear.
- Press OK button and restart Firefox.
- Type about:config in your address bar and press enter.
- An warning will appear. Press I’ll be careful, I promise! button
*** The above screenshot is from Firefox 25.