It's quite a rickety process ever since XBL was removed in 72. This repository is usually the preferred base if you're interested though https://github.com/xiaoxiaoflood/firefox-scripts
Yes we can still create uc.js in 2022. MDN no longer serve the XUL doc. However we can search their archived doc repo, e.g. https://github.com/mdn/archived-content/search?q=nsIPrefServ...
The tool I used before [1] https://github.com/satyr/uc hasn't been updated since 2016, and http://userchromejs.mozdev.org/ is entirely gone.