Right now search only pays if you do advertising. Mozilla's been using search ad revenue from Google and now Yahoo for revenue. If Mozilla gets into the search engine game they'll be getting in to the advertising game. And the advertising game is about serving up your users and what you know about them to the highest bidder.
That doesn't sound like a good fit with Mozilla.
[0] http://video.foxbusiness.com/v/4578856/mozilla-ceo-on-the-la...
Edit: After reading your history, I am astonished to see yahoo advocating strongly for online privacy. However, Yahoo is a profit oriented corporation, so I am naturally suspicious.
http://m.theinquirer.net/inquirer/news/2329566/marissa-mayer...
Adblock Plus to be bundled with Firefox in 5, 4, 3, 2…
Or, so I dream, anyway. But I’m sure that Yahoo is just as hungry for those tasty ad clicks (with their troublesome privacy implications) as Google is.