I don't know why you think this is astonishing. Google spends way more than that on Chrome, for what it's worth. Anyway, $400 million a year, if you spend it

I don't know why you think this is astonishing. Google spends way more than that on Chrome, for what it's worth. Anyway, $400 million a year, if you spend it all on developers (salaries, payroll taxes, benefits, desks, monitors, etc) gets you at most 2000 people in the US. And that's if you really stretch things, honestly. That assumes you don't need a continuous integration infrastructure (which is actually a pretty large cost in practice). And that you don't need legal, payroll, marketing. Or any project management or management at all. And that you're not doing any documentation or anything like that. In practice, Mozilla has a bit over 1000 employees and isn't spending all its revenue (so pays taxes on whatever it doesn't spend, and saves what's left).