Assuming they walk away from Yahoo, though, whom would they select as their next US search partner? Return to Google?
Same goes for the IE team: http://thenextweb.com/insider/2012/10/27/mozilla-sends-the-i...
For both Google and Mozilla, the goals of these two (or more!) groups of decision makers will not necessarily align at all times. Ideally, whatever may be going on at the ad/search/business level shouldn't impact collaboration on web standards and implementation.
With $1 billion in cash guaranteed, they could even try DuckDuckGo for a couple years and see if DuckDuckGo could grow enough to support them.