It is a website to security harden modern web browsers. It gives recommendations based on the level of security you need, but otherwise you can freely choose the options you like. It currently supports:
It is written as a single page web application, which means you can just clone this reposity and open index.html - no webserver is needed! You can also try out the online version at https://harden.it
The user.js file contains all the settings you have set. Move it into your Firefox profile folder and restart Mozilla Firefox. For more information, see https://github.com/pyllyukko/user.js/#installation