I'm an American who worked in Germany. I interviewed in the the Netherlands, but was horrified by how low pay was in the industry.

Its speculation. Cost of benefits/rent/etc is expensive in the Bay Area, but no one is getting $25k worth of dental insurance or anything.

It's just an estimate for an based on my experience in the industry, but I think it's a reasonable one. I did estimate based on American costs though. For example, an entry-level software engineer at Google in the U.S. makes $188,000 according to https://www.levels.fyi/ . That's the amount of compensation that goes to the employee, though; that doesn't count employer taxes and the additional costs of overhead like office space. So I added a range of up to twice that much to account for overhead, to estimate what an employee costs to the employer.

There are a couple other factors - one extra factor is this is just for entry-level employees, another extra factor is that Mozilla probably doesn't pay quite as well as Google. I am not sure how large those factors are, but it's just an estimate and they work against each other, so I think the range is a reasonable guess. I would be surprised if it's very far off.