A user.js is a configuration file that can control Firefox settings - for a more technical breakdown and explanation, you can read more in the wiki
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. There is also an interactive current release, thanks to icpantsparti2.
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.
Literally thousands of sources, references and suggestions. Many thanks, and much appreciated.