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 the Mozilla Add-ons directory contains a massive library of extensions and themes to add additional functionality and make Firefox reflect your own needs and personality.

Two additional forms of advanced configuration allow even further customization: about:config and userChrome.css modifications. However, Mozilla highly recommends 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 that future updates won’t impact these customizations. For this reason, Mozilla does not officially support setting custom style rules with a userChrome.css file or using the about:config configuration editor to change preference settings.

If Firefox suddenly appears broken after an update, you may need to use the Refresh feature to restore Firefox to a default state while keeping important data. Alternatively, you can reset preferences in about:config or you can delete the preferences file(s) from your profile folder, to restore preferences to default settings. To remove customizations made in the userChrome.css file, you can delete, rename or remove that file.