Earlier quoted context omitted.
Last time I checked, Mozilla's ARPU was less than $5 pa. I think many of us would pay a multiple of that per annum _iff_ it went towards Firefox and not whatever project/cause of the week that Mozilla has undertaken.
You're overestimating people's willingness to pay for software when free and arguably better alternatives are available. Preferring Firefox to free Chromium alternatives requires a level of nuance and tech literacy that most people will never have, and even with that tech literacy, people may still prefer Chromium. You're basically talking about asking for donations from people that prefer to ad-block YouTube instead…
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.