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Honestly, the region shouldn't matter. If you want you could add a little notice that it's English-only for now, but there's absolutely no reason to limit this to a particular country. It's a totally free service and it doesn't involve complicated region-specific licensing agreements, so where someone lives quite simply does not matter.

All English speaking countries would be a good start for (almost?) zero effort on Mozilla's part.

I hope you are aware of the fact that language and geography most of the time don't match because one person is capable to speak/read multiple languages and that English is understood by people outside of the USA?