Google and Mozilla are not enemies. They are both allies towards an open web. Sure there's the media-hyped browser wars, but Mozilla is about freedom of choice on the web. They are a non-profit who uses Firefox as a playing card in order to further the web. That's how it all started. Microsoft halted development of IE because they saw the web as a threat and Mozilla stepped in. It's never been about the money.

Besides, taking on Google, who accounts for over 90% of Mozilla's cash flow is foolish, especially when Mozilla is a team of 650 and Google is a giant of 30,000 (not to mention in a totally different product). But that's besides the point.