500 million could net you about 50 to 70 million annually if you put it all on the S&P 500... A few years of this and you're a self-funded non-profit...
Reminder: Ladybird is being developed by a handful of people with contributions from the community. It's far from being complete, but it clearly shows that you don't need an enormous budget to build a Web browser entirely from scratch, let alone maintain one.
They're doing that. Their endowment is invested and it does grow by a non-trivial amount, though I don't know the exact numbers off the top of my head.