Weird idea - since these big companies all voluntarily stopped documenting in favor of MDN, perhaps they should pay Mozilla for this (it's saving them money, after all).
If Mozilla is still having to do a paid option - MDN Plus - they are obviously not getting enough. Looks like around the price of one employee for the top funders.
Unless Mozilla is suddenly operating for profit and following the ol' "why get some of the money when you can get all of the money" business model.
You're thinking of the Mozilla Foundation, which is not-for-profit. The Foundation owns the Mozilla Corporation, which is "for-profit" and develops Firefox and MDN.
Money flows from the Corporation to the Foundation every year.