It would be organizational suicide to spend down their endowment just because they can. Right now it exists as a firewall to buy them some time in the event that search licensing goes away, which I think is exactly what they should have done with it.
And it's been talked to death before but the idea that the browser side bets are at some prohibitive cost is an unsubstantiated myth, conjured into existence by vibes in comment sections. It's the HN equivalent of American voters who think foreign aid is 50% of the federal budget.
Do you realize what 1.300.000.000$ is? Say you invest most of it in a safe way to get you inflation + 2%. That gives you 26.000.000$ every year. You can pay 100 engineers with this. Firefox is a browser. Sure a browser is complicated but 100 motivated and talented engineers is more than enough to make a good product if you focus on what matters.
There is no excuse to what is going on.