Google doesn't really make money from Firefox or Chrome (the browser), per se (except insofar as Chrome increases the value of things that Google does sell that include it, like Chromebooks, Nexus devices, etc.), it makes money from searches, whether default or not. 3 years ago, sure, more searches -- to Google or any other search provider -- came from Firefox than Chrome because Firefox had more usage share than Chrome. That's rather dramatically reversed since then.