Firefox advanced customization and configuration options
https://support.mozilla.org/en-US/kb/firefox-advanced-customization-and-configuration
WebTo 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. Advanced Firefox customization options, about:config and userChrome.css/userContent.css modifications, are not recommended.
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