Sets Firefox/Thunderbird Locale to Windows (MUI) Language on startup. This is most useful in Multi-Language Environment. If you install fx/tb language packs for languages that are installed on the OS, the app language follows automatically. This behavior can be overridden, if the user selects the language explicitly with the "locale switcher" extension.
This extension will not be ported to Webextensions. With FF57, the pref "intl.locale.matchOS" works now as it should on the Windows-version of FF (thanks, Zibi!). So, there is finally no need for this extension anymore.