Google gives money to Mozilla (and to Opera). They basically pay for users using the search. They want to avoid the risk of Microsoft (or anyone else really) paying more than they want to afford. I think Opera had bing as a default search in some of the recent versions. Also, with Chrome they encourage users to search more (instant search) => more ads, more money.
Oh yes, I totally forgot Instant Search. So if I understood you correctly, Google gives Mozilla money to set the home page to Google.com and not the other way around. I guess that makes more sense that way.
