A bug report opened about nine months ago on Mozilla's Bugzilla bug tracking site for Firefox suggests that the organization could disable reading the user.js file of the Firefox browser by default in the future.

If you have not heard about user.js before, it is a configuration file that controls preferences in the Firefox web browser. One of the main advantages over Firefox's preferences file is that it has priority and that it is a user-owned file that is left untouched when Mozilla makes changes to the browser.

I suggest you check out the ghacks user.js repository on Github https://github.com/ghacksuserjs/ghacks-user.js/ for detailed information and an extensive file to improve privacy and security of the Firefox web browser.

https://www.ghacks.net/2020/01/06/please-mozilla-dont-touch-the-user-js-functionality-in-firefox/