While I don't think user payments would be enough to fund development, I don't think "they need Google's money" is evidence for it. It's not like paying for Firefox is currently an option.

Firefox is developed by the Mozilla Corporation. The closest thing to paying for Firefox is probably donating to the Mozilla Foundation, but that money is used for education and advocacy.

Perhaps, but also consider that they are only in this position because Google uses their search monopoly to fund development of a free alternative... much like Microsoft used their desktop OS monopoly to do the same in the 90s.