Version at time of writing: Firefox 79.0 First, locate your profile by going to the firefox menu bar: Help -> “Troubleshooting Information” and find on the page “Profile Directory”. Navigate there, where you’ll be making/adding two files that allow you to customize behavior and appearance of Firefox. For reference, mine was here: ~/.mozilla/firefox/aabkj82t.default/ . Yours will have some other blob of random characters before “.default”. userChrome.css This file should definitely exist already.