My impression is that Google is simply having to play the same game their own advertising business makes others play; in order to keep getting customers (eyeballs for adsense+doubleclick) they have to pay for it because the competition also advertises (via paid default search).
Firefox switching to Yahoo for default search was probably an eye opener for them; 30% of global search traffic (at whatever fraction of users didn't change their search engine) is a boatload of money and from public estimates at least an order of magnitude more than they pay Mozilla for the traffic.
From a business perspective Google might benefit if Firefox disappeared but there's no guarantee. Opera, Brave, Safari, Edge, etc. could take up Firefox's install-base without significantly increasing Chrome's. At least Firefox is a known entity for them and they would probably rather have a guaranteed platform with Google as the default search engine than having to negotiate with whatever sprang up to replace Firefox.
