No, they definitely could go back to Google. Problematic is just that while Google will probably pay, there's now essentially a third of the competition for that spot gone, so Google will have to pay significantly less.
Never mind also that Chrome has made leaps and bounds. I don't know the details of the history of Mozilla and Google's partnership, but featuring prominently in a major browser was probably a lot more valuable to Google back when Google didn't have a browser of its own.
It is also a bit of a conflict of interest on Google's part to support Mozilla. They have a competing app ecosystem, and while supporting the Firefox Marketplace and Chrome App Store are actually relatively simple for HTML5 apps, they probably would rather not have the competition at all.
