That isn't really the best way to think about not-for-profit schemes like Mozilla. Every organisation eventually becomes corrupted (as in fact we see with Mozilla), so creating an eternal pot of money for something is not strategically sensible.

If good people are in charge, they'll just spend everything and rely on ongoing donations. If nobody thinks it is worth donating too then it is time to close up shop. Keep a bit of a buffer for the practical issue of bad years, sure, but the idea shouldn't be to set up an endowment.