I think it helps somewhat that, for different reasons, this is not really about the money for either one of them: Firefox is a nonprofit organization, and Chrome is not Google's main product.
In some ways it is about money, Google pays Mozilla about $300 million a year for search engine placement and click-through ads. That accounts for the vast majority of all of Mozilla's revenue (in 2010, Google paid $115 million and that was 85% of Mozilla's revenue).
It might also help a little that Mozilla depends mostly on Google for revenue :-) ...but google has stated before that basically "anyone in favor of improving the web is on our side"
I think it makes it all the more admirable, though arguably Chrome benefits Google pretty well. They build better apps. You spend more time on their property viewing their ads. They build more HTML5 functionality that makes their ads even more engrossing. They get your homepage, etc.
