I disagree with the idea that it is unprofitable to not support competing browsers. Just look at United States v. Microsoft Corp.
But, as you say, Google is first and foremost a business. As a business, it has a process where, if you don't have impact, you get fired. There are lots of people who, if forced to choose between "deferring work on Firefox bugs" and "getting fired", would choose the former. I would rather that they didn't have to make that choice in the first place.
And FWIW, I recognize that the engineers are usually not the same as the businesspeople who make the business decisions. :)
Those billions of dollars don't code for themselves. You need to hire more folks (yes, overtime is a thing at Google), you need to direct them to do the right things (means: more hiring), and more infrastructure. Besides the cafeterias which boil down to "even more hiring" (and why not, we did that twice in our thought experiment already) those devs need office space (of some kind), equipment and server time for testing their thousands of additional commits every week. And more server time to cover the additional test cases (eg. for additional browsers), so this is super-linear.
Given infinite dollars (or any reasonable approximation of that), the problem really is how to spend them without ending up finishing nothing because you've just invested them in a giant gridlock situation.
Clearly, they have enough money to "improve the lives" of more people, so that answer doesn't satisfy. However, the idea that Google is first and foremost a business which makes money at any expense, and it isn't profitable to support a competitor's browser, does.