This isn't the first time a company funded its competitor to avoid monopoly accusations. Microsoft did it to Apple. So it's not like Google is simply returning the wealth Mozilla is providing it out of some generosity. Maybe they are, but I find the desire to remain out of the clutches of regulators to be an equally compelling explanation.

And given the fact that (despite Mozilla's best attempts to the contrary) Firefox users tend to be on the nerdy and privacy oriented side, and they have both the proclivity and capacity to block ads, I imagine that Google probably pulls from the revenue sucked out of Chrome users rather than Firefox ones. But that's just a theory, a browser theory.