Maybe consider to use the mozilla.cfg file lock set this pref. *intl.accept_languages This is a complex pref (can be localized) and thus requires special treatment to set the default value. *https://mike.kaply.com/2012/03/15/customizing-firefox-default-preference-files/ *https://mike.kaply.com/2012/03/16/customizing-firefox-autoconfig-files/ You can use the <u>mozilla.cfg</u> file in the Firefox program folder to set or lock preferences and run privileged JavaScript code. See Configuration: *https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment

Thanks a lot!!!