Steps to Reproduce: 1. Install Czech version of Fedora. 2. Open pre-installed Firefox. 3. Open about:config and find "general.useragent.locale". Actual results: The value "cs-CZ" is non-standard (Mozilla builds have "cs" only, respectively for all languages use two character codes unless there are more regions). Consequently is breaks the usage of WebExtensions i18n API [1], returning value that is not expected by extensions. Expected results: The "general.useragent.locale" should be the same as for builds made by Mozilla and correspond to the Mozilla locale ("cs" in case of Czech). Mozilla builds and languages: https://product-details.mozilla.org/1.0/firefox_primary_builds.json https://product-details.mozilla.org/1.0/languages.json Additional info: [1] https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/i18n/getUILanguage
Turned out to be a bug in localization upstream (in toolkit/chrome/global/intl.properties).