While I believe the party line is probably true ("Mozilla decided not to go with Google"), it's also disingenuous.
Mozilla chose not to go with Google because Google wasn't willing to pay what they were before. They straight up told Mozilla this 3 years ago when they signed the billion dollar contract; Mozilla had 3 years to become profitable. That's why they switched focus to FirefoxOS; they thought that by now they'd be profitable via selling phones and the app store. (At the time, Bing was bidding against Google, however Mozilla went with the smaller check from Google because they knew using Bing would seem like selling out.)
For the record, Google made billions off being Firefox's default search engine. They paid Firefox $300mil a year for three years, but that was only a small fraction of how much Google profited from Firefox searches. Not sure if it's still true, but three years ago they made more from Firefox than they did from Chrome.
So, yes, Mozilla could have gone with Google still. It's not like Google said "nope, you can't use us as the default!". However, they went with Yahoo! because Google wasn't willing to pay what Mozilla needed. The whole "Mozilla picked Yahoo! to enable choice" has been tweeted by every Mozillian I know (and said multiple times in this thread), but it's a meaningless statement. If they really meant that, you'd be prompted when you opened Firefox the first time to pick a search engine.
http://thenextweb.com/apps/2012/11/12/opera-users-click-on-t...