From #33534: Firefox 76 enabled the following: - numberingSystem and calendar options for the `Intl.NumberFormat`, `Intl.DateTimeFormat`, and `Intl.RelativeTimeFormat` constructors - the `Intl.ListFormat` API Firefox 78 added support for `Intl.DisplayNames`. It looks like `RelativeTimeFormat()` may leak the current locale if the caller passes undefined to the constructor instead of a locale string. Other APIs may have similar problems that could make it easier to perform fingerprinting based on the user's locale. https://bugzilla.mozilla.org/show_bug.cgi?id=1625975 \ "Enable numberingSystem and calendar options by default" https://bugzilla.mozilla.org/show_bug.cgi?id=1589095 \ "Add support for "type" and "style" options to Intl.ListFormat and enable Intl.ListFormat by default" https://bugzilla.mozilla.org/show_bug.cgi?id=1557727 \ "Implement the Intl.DisplayNames proposal"