Firefox advanced customization and configuration options

Firefox supports a wide range of customization options that are well-suited to all skill levels. The [[Customize Firefox controls, buttons and toolbars|toolbar layout can be customized]], numerous [[Firefox options, preferences and settings|settings can be configured]], and the [https://addons.mozilla.org Mozilla Add-ons] directory contains a massive library of [[Find and install add-ons to add features to Firefox|extensions]] and [[Use themes to change the look of Firefox|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 {filepath 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 [[Refresh Firefox - reset add-ons and settings|use the Refresh feature]] to restore Firefox to a default state, while keeping important data. Alternatively, you can try to selectively reverse your advanced customization and configuration changes: *To reverse changes you made to Firefox with a {filepath userChrome.css} file, you can use ''about:config'' to [[Configuration Editor for Firefox#w_resetting-or-deleting-preferences|reset the preference]] {pref toolkit.legacyUserProfileCustomizations.stylesheets} to {pref false} and also remove (or rename) the {filepath userChrome.css} file located inside the {filepath chrome} folder in your Firefox [[Profiles - Where Firefox stores your bookmarks, passwords and other user data|profile folder]]. *To reset only preference modifications, you can either use ''about:config'' to [[Configuration Editor for Firefox#w_resetting-or-deleting-preferences|reset all modified preferences]] or you can delete the preference file(s) that saves preference changes from your [[Profiles - Where Firefox stores your bookmarks, passwords and other user data|profile folder]], as explained [[Reset Firefox preferences to troubleshoot and fix problems#w_solution-2-delete-firefox-preference-files|here]], to restore all preferences to default settings.

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 try to selectively reverse your advanced customization and configuration changes:

• To reverse changes you made to Firefox with a userChrome.css file, you can use about:config to reset the preference toolkit.legacyUserProfileCustomizations.stylesheets to false and also remove (or rename) the userChrome.css file located inside the chrome folder in your Firefox profile folder.
• To reset only preference modifications, you can either use about:config to reset all modified preferences or you can delete the preference file(s) that saves preference changes from your profile folder, as explained here, to restore all preferences to default settings.