They could've at least tried to sell a paid version - what's the worst that could happen? Any sale would be on top of what they're currently earning per download, i.e. pure "profit" that could be reinvested in the product.
It never was an "either/or" proposition.
Assumedly, a paid version would exclude some features that Mozilla is otherwise monetizing through (like selling your data). This doesn't seem like sales "on top of" what they're already earning, but rather an alternative that replaces (at least some of) existing monetization routes.
But indeed, even if they ensure that donations go towards Firefox development, instead of other crap, that would be a step forward.
We have no interest in funding Mozilla, whose manifesto barely mentions Firefox and who has now decided that AI is their focus.