Apple would because nowadays Google is a more direct competitor than Microsoft is. Mozilla, I would consider far less likely; they're less profit-driven, more ideological, Google aligns more ideologically (although not completely), and Microsoft and Google are about equally direct competitors.

More money?

Also, Apple might be strategically happier working with Microsoft than Google right now.

Google has Mozilla LOCKED in through 2011.

Mozilla makes $50 - 80 million a YEAR from their search referrals.

But in context, MSFT has tens-of-billions in CASH sitting around, and is trying to win the internet space.

If they did a 1.5x revenue payout vs google (keeping in mind that's not all cash lay out, they net gains from use of their search engine) and dropped a billion in 5 years I would vote for that as a shareholder.