Firefox advanced customization and configuration options

Firefox supports a wide range of customization options that are well-suited to all skill levels. The toolbar layout can be customized, numerous settings can be configured, and our Add-ons directory contains a massive library of extensions and themes to add additional functionality and make Firefox reflect your own needs and personality.

There are two additional forms of advanced customization that allow even further customization: about:config preferences and userChrome.css modifications. However, we highly recommend that only the developers consider these customizations, as they could cause unexpected behavior or even break Firefox.

Firefox is a work in progress and to allow for continuous innovation, Mozilla cannot guarantee future updates won’t impact your customizations. For this reason, Mozilla does not officially support custom style rules or the configuration editor.

If your Firefox suddenly appears broken after an update, you may need to restore your Firefox to its default UI, reset the preferences configured in about:config or reset your Firefox preferences.

Learn more about using and troubleshooting the about:config preferences.