Go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true
Add userChrome.css to ~/Library/Application Support/Firefox/Profiles/<PROFILE_ID>/chrome/userChrome.css
Ref:
- https://developer.mozilla.org/en-US/docs/Tools/Browser_Toolbox
- https://support.mozilla.org/en-US/questions/1157451
- https://www.ghacks.net/2019/05/24/firefox-69-userchrome-css-and-usercontent-css-disabled-by-default/
- https://superuser.com/questions/1495834/is-a-left-side-tab-close-button-in-firefox-possible
