Firefox 57, also known as Firefox Quantum, has a fresh look. In addition to the built-in Customize feature (see Customize Firefox controls, buttons and toolbars at Mozilla Support), you can modify the appearance of Firefox using the powerful userChrome.css feature.
userChrome.css holds style rules for modifying Firefox's user interface. You can find users on the web offering a wide range of ready-to-paste style recipes, and communities where you can request help with rules to make your Firefox look just the way you want. And it's all free.
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This site is not maintained by or affiliated with Mozilla, the maker of Firefox. For more information on Firefox, please visit: phototelescope.