The user.js is a configuration file based on the arkenfox user.js profile for firefox.
If you don't know what im talking about, check: https://github.com/arkenfox/user.js
The intent of this fork is to find a middle ground for most people between convinience and privacy on the user.js overrides (tending more to privacy).
The arkenfox 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 wiki, as it contains important information regarding a few user.js settings.
Note that we do not recommend connecting over Tor on Firefox. Use the Tor Browser if your threat model calls for it, or for accessing hidden services.
Also be aware that the arkenfox user.js is made specifically for desktop Firefox. Using it as-is in other Gecko-based browsers can be counterproductive, especially in the Tor Browser...