Wrong Mozilla. Mozilla Corporation is developing Firefox (and giving its profits to the parent entity, Mozilla Foundation), the foundation isn't paying for Firefox development.

It has been confirmed many times that exactly zero of funds donated to the Mozilla Foundation go to Firefox, which is developed by the Mozilla Corporation.

There are currently 220 million Firefox users[1]. If 10% of those paid $2 per month, it would be more than $500 million per year. Their Google deal is worth about $400 million in good years.

I imagine it would be fewer than 10%, but even the low single digits would be enough to run a slimmed-down company focused on Firefox alone.

[1] https://data.firefox.com/dashboard/user-activity