Alternatives to Firefox are free. If this software has to survive than enough people will have to pay either directly or indirectly as in case of ads.
What are other ways that an open source software can survive that cannot have "Open core" or "On premise" options available to it?
Ask for money? Hell there's still not actually any way to pay for Firefox. Even this ad doesn't actually allow you to pay for Firefox, but instead for a service that's useless to the vast majority of people and relies on fear-mongering to sell itself.
What I'd actually pay for is something more elaborate like icloud private relay. Unfortunately Mozilla just cancelled that and it was only in beta in the US.
I'd also pay for Firefox itself if I could (I already donate to the other projects I use a lot like KDE) but I feel a bit disenfranchised with Mozilla still aiming very hard at the mainstream user they have long lost.