Steps to Reproduce:
1. Install Czech version of Fedora.
2. Open pre-installed Firefox.
3. Open about:config and find "general.useragent.locale".
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 , returning value that is not expected by extensions.
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:
Turned out to be a bug in localization upstream (in toolkit/chrome/global/intl.properties).