AFAIK it's more correct to say that only some particular countries are excluded, but I'm not HR and I have no idea where your are getting that impression from, so I won't comment further.
This is quite far removed from "remote hiring limited to some particular countries". A large part of Mozilla's workforce is spread out through Europe, so there can't be too many exceptions.
A very small shop in a very beaurocratic EU country receives the invoices in PDF at the end of the month, from another country far outside the EU, pays the bill through standard bank wire transfer. Why Mozilla can't do the same?
