You're basically talking about asking for donations from people that prefer to ad-block YouTube instead of paying for Premium.
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.
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.
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.
The people who care are willing to pay.
I think it's already the case that only tech literate people prefer Firefox over Chrome or Edge (I bet a significant part of users don't even know about Chromium or what's the difference from Chrome). So putting a price on Firefox wouldn't change this in a meaningful way. The real question is how much tech literate people would be ready to pay, most of the users will stay on Chrome/Edge for the foreseeable future.