Mozilla.org is the corpse of Netscape that Google keeps animated so that it looks like they have competition when they really don't.

The existence of Firefox is something they can point to to say they're not a monopoly. The fact that 80% of the revenue Firefox receives is from Google means that Google effectively controls them. Mozilla has to weigh every decision against the risk that it will cause Google to withdraw their funding. That severely restricts the choices they're willing to consider.

Firefox is only 5% of browsers, so it really doesn't matter to Google if that 5% of users considers using a different search engine. Because of the Firefox user base, many of them will have already switched search engines, and because Google is such a dominant player, many others would switch back to Google if the browser used a different default. So, maybe 10% of that 5% would permanently switch search engines if Google stopped paying. Is that really worth billions per year? Probably not. But, pretending like you have competitors in the browser space and using that to push back on antitrust, that's definitely worth billions per year.

Google pays Mozilla in exchange for google being Firefox's default search engine

For an example, Mozilla being forced to use Google Location Services as default even though Mozilla has its own. I am also a Firefox user but it always makes me wonder what other TnCs forced on Mozilla as part of the search deal.