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.