I'd add that fundraising has worked well for the Wikimedia Foundation. They're taking in around $175M/year via donations. That isn't the nearly $500M/year that Mozilla gets from Google, but it's still a ton of money.
I don't know if people will donate for their browser like they donate for Wikipedia, but if it's able to bring joy to people, it could be pretty sustainable. Even Mozilla takes in $10M/year in contributions.