Before you have a whinge about that idea, realise that google effectively does this already.

All it would take is for Firefox to say fuck it, you want to serve ads to our users? Fine, but you sell adunits through our network that is focused on privacy, not the network through Google or Facebook.

When? It's not pragmatism anymore, it has become pure laziness on the part of Mozilla. They have been accepting this business model (being fed by the hand of Google) forever now, instead of seriously trying to achieve financial autonomy. They have lost all credibility if they compromise on that front - it's such a huge conflict of interest.