Mozilla decided[1] to use GENC[2], a list produced by the US Government and including additional countries that are not correctly covered by ISO3166.
The US Govertment's approach (making their own list based on ISO3166) is similar in some ways to the approach I have suggested for Debian. 1. https://bugzilla.mozilla.org/show_bug.cgi?id=733417#c20 2. https://nsgreg.nga.mil/doc/view?i=2500
