user.js is a configuration file that can control hundreds of Firefox settings. For a more technical breakdown and explanation, you can read more on the overview wiki page.
ghacks user.js is a template which aims to provide as much privacy and enhanced security as possible, and to reduce tracking and fingerprinting as much as possible - while minimizing any loss of functionality and breakage (but it will happen).
Everyone, experts included, should at least read the implementation wiki page, as it contains important information regarding a few
ghacks user.js settings.
Also be aware that this
user.js is made specifically for Firefox. Using it as-is in other Gecko-based browsers can be counterproductive, especially in the Tor Browser.
Literally thousands of sources, references and suggestions. That said...
1 The ghacks user.js was an independent project by Thorin-Oakenpants started in early 2015 and was first published at ghacks in August 2015. With Martin Brinkmann's blessing, it will keep the ghacks name.
git clone ghacksuserjs-ghacks-user.js_-_2019-04-19_05-54-58.bundle