my configuration for i3
Adapted instructions from https://github.com/MrOtherGuy/firefox-csshacks
Go to about:config and set the pref toolkit.legacyUserProfileCustomizations.stylesheets to true
While Firefox is running, go to about:support and find a Profile folder row near the top - there should also be a button labeled "Open folder" next to it. Clicking that button should open the folder in your file manager.
The profile folder should have files like prefs.js and places.sqlite
Firefox loads userChrome.css from /chrome/userChrome.css. That chrome-folder or the stylesheet files do not exist by default.
Manually copy individual styles directly into userChrome.css