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Two types of advanced Firefox customization options, about:config and userChrome.css modifications, are not recommended. Learn more.

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 other forms of advanced configuration allow even further customization: ''about:config'' preference modifications and ''userChrome.css'' or ''userContent.css'' custom style rules. 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 adding custom style rules with a {filepath userChrome.css} or {filepath userContent.css} file, or using the ''about:config'' [[Configuration Editor for Firefox|configuration editor]] to modify 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 selectively reverse your advanced customization and configuration changes, as follows: *To reverse changes you made to Firefox with a {filepath userChrome.css} or {filepath userContent.css} file, use ''about:config'' to [[Configuration Editor for Firefox#w_resetting-or-deleting-preferences|reset the preference]] {pref toolkit.legacyUserProfileCustomizations.stylesheets} to {pref false} and then remove (or rename) the {filepath userChrome.css} or {filepath userContent.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 all preference modifications, either use ''about:config'' to [[Configuration Editor for Firefox#w_resetting-or-deleting-preferences|reset all modified preferences]] or else delete the preference file(s) in 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.