There's nothing to acknowledge. You're asking everyone to accept the presumption embedded in the statement that a billion dollars "goes away quickly when you're a large company paying lots of money to salaries", namely that Mozilla should be a large company and should rely on a steady stream of outside money instead of seeking sustainable financial independence. But Mozilla's lack of focus and excessive spending on side projects is a major part of the complaints against Mozilla, and you aren't even trying to make a reasonable case that Mozilla needs to be spending money like that.
Does their endowment fund enable them to be an independent and self-sufficient entity?
In other words, Can they live off it in perpetuity?