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 selectively reverse your advanced customization and configuration changes, as follows:
- To reverse changes you made to Firefox with a userChrome.css file, use about:config to reset the preference toolkit.legacyUserProfileCustomizations.stylesheets to false and then remove (or rename) the userChrome.css file located inside the chrome folder in your Firefox profile folder.
- To reset all preference modifications, either use about:config to reset all modified preferences or else delete the preference file(s) in your profile folder, as explained here, to restore all preferences to default settings.