- csshacks: https://github.com/MrOtherGuy/firefox-csshacks
Go to about:support and find a Profile folder row near the top. Firefox loads userChrome.css from <profileFolder>/chrome/userChrome.css.
Use Command + Opt + Shift + I to open the global inspector.
Ref: Create your own firefox css: https://www.youtube.com/watch?v=bw_M7q3Mtag&t=322s