Firefox has a config file which can be accessed by typing about:Config in the url section. This file is similar to Windows registry file as it contains information on the running of Firefox.
The config file contains lots of information which the users can change apart from the default value. But changing this value can also be dangerous in the sense that it can sometimes cause Firefox to stop working. So its better to take a backup of the settings file, so that in case you change any value and any error occurs, you can always restore Firefox back to normal state.
How to take the back-up: Taking a back up is simple process. Select Run in Windows and type “%appdata%\mozilla\firefox\profiles“. It shows a Windows folder which contains a folder which has a name like ati9z0wb.default (the name may not be same in all computers). Open the folder and then copy the file prefs.js to a safe location.
Should you mess something up in about:config file, simply close Firefox and copy the backup of prefs.js back to its original location.