Check that Intl.RelativeTimeFormat does no leak the user agent locale
https://bugzilla.mozilla.org/show_bug.cgi?id=1504334 is shipping the new Intl.RelativeTimeFormat
interface.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RelativeTimeFormat has some more information about this feature which appeared live in Firefox 65