It would be possible, but it would involve some major legal wrangling, and there's a good chance that it wouldn't pay for itself and/or would tie Mozilla's hands in problematic ways.

The best route still appears to be paying for whatever paid services Mozilla comes out with (VPN, Pocket, ...?) even if you don't use them. I don't know if a "pay what you like" option would be problematic?

(Regulators watch pretty closely when you have 100s of millions of dollars of annual income.)