They have ads to fundraise. Wikipedia is one of the greatest archives of knowledge in history. Their clients and website are open source powered by MediaWiki. Of all the sites to use a privacy friendly frontend for, I'd have Wikipedia at the very bottom.
This is really exciting, Mozilla is one of the few companies I trust with something like this. That being said, many initiatives spearheaded by the Mozilla foundation end up in failure, often because of the lack of funding. I hope that the community can help get the ball rolling on this, even if Mozilla is not ultimately there at the finish line.