It's great if you can get your endowment so big you never have to worry about revenue but outside of, say, elite universities or middle eastern sovereign wealth funds that rarely happens.
2. 1 billion dollars is a lot of money. Even the interest off it is huge.
3. 10 years is a very long time in tech.
4. I would greatly prefer the money Mozilla earned due to Firefox being a thing was put into developing Firefox, yes. The current Mozilla organization seems to be a mechanism for providing third homes for the executives, starting projects nobody wanted them to start, sullying the Firefox brand with them, and then abandoning them. There's a VC cancer infesting the supposed "free software community" called Mozilla.
Wait, what? I thought your whole premise from one comment ago was that they "stop doing everything" and exclusively slow burn away their endowment. They're dead by 2029 if they do that.
If they don't do that, then you're just talking about how they currently operate.
Mozilla should stop doing all these side quests -- look at their track record! -- and they should get rid of the fat executive layer. They should transparently report what they're using their money, instead of saying they burn hundreds of millions of dollars in "software development" while firing the Servo developers.
I don't think actual competition benefits Google in a commercial sense. If we considered the situation purely rationally, Google's most logical decision would be to use their budget as leverage, threatening off the books to prevent any strategy that might make Firefox "viable again", under the assumption that Google focused primarily on market share, while Mozilla focused purely on survival.