prefs.js is modified by browser itself. And it contains lots of stuff by default already. You can store your custom preferences in user.js file - Firefox will

prefs.js is modified by browser itself. And it contains lots of stuff by default already. You can store your custom preferences in user.js file - Firefox will copy those to prefs.js at startup. From your link: The user.js[1] file is optional. If you have one whenever the application is started it will overwrite any settings in prefs.js with the corresponding settings from user.js. [1] https://kb.mozillazine.org/User.js_file https://kb.mozillazine.org/User.js_file