Google wants Firefox to remain viable (or at least one other browser) so that they can avoid monopoly issues with Chrome. If they pay Firefox to keep Google as default search engine, they keep 80% of the money they’d get by having those users use Chrome, and they keep the other browser alive, but not enough to really keep up with Chrome’s feature set.

That’s the most likely bet they are making, similar to Apple/Android or Safari/Chrome. Spending a minor fraction of your revenue to avoid anti-trust probably makes sense for them.