They should ask (not require) payment for that service. Yes, what they should do is basically to run a Patreon subscription, clearly marked "For the continued development of Firefox".
At a very accessible level of $3 / mo on average, and merely 5M subscribers over the world, they'd have $15M monthly, or $180M a year.
See how Wikimedia Foundation is basically flush with cash, without getting handouts from its competition (haha), zero ads, and asking users to spare $2.75 here and there.