I think the real question here, aside from how to displace Android and IOS, is: how do the developers get paid for the upgrades, new features, security analysis and fixes, developing new boards and coming up with bsp (board support packages, essentially a "distribution" for hardware manufacturers that works on whatever new boards they relase) and infrastructure of such an OS.

Let's just assume this is about the amount of effort Mozilla puts in. So they'd need to collect ~500 million per year.

Where does that money come from? Presumably the answer can't be Google.