They're a charitable nonprofit, and the Mozilla license is one of the more permissive ones; they're fine with you freely sharing their work.
They currently already get 7M/year in donations for no purpose. I imagine they'd get a lot more if that money would fund Firefox, and how many core/paid developers do they really need if they have people that know that they're doing?
Mozilla also doesn't have to operate out of one of the most expensive cities in the world. And no, they don't have to be there to attract competent developers either.
If even a Microsoft Executive is saying that anything on the web can be freely copied then what does a normal user think.