They advertise remote jobs but it is actually limited to several counties. Also, they differentiate salaries based on your location.
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.
I'm not sure what the industry standard is wrt adjusting salary based on location, but considering the cost of housing in the Bay area or NYC, it seems fair to pay the folks who live there more than, for example, me who lives in the boonies of Florida.
It's true we don't hire everywhere. Most people are from the EU, United States, and Canada. There is an office in Japan, China, Taiwan, and a small group in New Zealand, with some people scattered about in Australia. The timezones in Asia are really rough, so I believe there's reluctance to hire there for positions where the team is not already located in Asia.
Outside of those countries I think we've used Elance (which is Upwork now?), but I think that's discouraged for new hires.