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.
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.
Given two otherwise equal tasks, if one helps 60M users and another helps 12M users, you are usually expected to work on the one that helps more users. And if a product develops too many bugs of the latter type (affects too few users to ever be prioritized above other work), it will usually get shut down.
Fixing bugs for one or two or even a hundred users doesn't scale, and Google wants to scale so it can live up to its "potential to improve the lives of millions of people". (If you ever work at or start an early-stage start-up, you want to be the opposite of this because Google can't; see Paul Graham's "Do Things That Don't Scale" essay.)
If you come back (for another internship or full-time), the Intern Host, Product Manager, Director, and/or VP Engineering in your management chain can provide more context on how to apply "put the users first" in your specific context.
There are times where I was expected to put IE 9/Win XP above Firefox because that would help more users. And yes, that sucks, both for me, and for Firefox users.