A 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.
The ghacks user.js is a template, which, as provided, aims, with extensions, 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 default settings we use. The rest of the wiki is helpful as well.
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.