Firefox used to be "the" customizable browser - where you could make it look the way you wanted.
Not anymore. Addons like that dont work anymore.
Not anymore. Addons like that dont work anymore.
And each update you have to adjust your CSS to fix whatever the UI changes broke. That is of course until Mozilla remove userStyle.css entirely - they have already disabled it by default so for new installs you have to toggle toolkit.legacyUserProfileCustomizations.stylesheets in about:config.