Mostly from search engines, especially Google. They also own Mozilla Corporation, a for-profit entity which reinvests all of its profits back into the Mozilla Foundation.
Well as a user of both opera GX and Firefox I haven't seen that many obnoxious links or anything of the like. But then again, I didnt see many when I was on chrome so who knows.
They get paid many millions by Google for having Google as the default search, ironically.
It was once a fortune but no longer is as they don't have many users to send to Google.
In recent years they've been trying to find alternate income streams, most recently re-branded mullvad VPN. None of them have worked. There's been a lot of criticism, it's not simple but the case can be made that they have squandered their wealth.
When you open a new blank tab you can have like 10 pinned websites in the middle. Sometimes 1-2 of them are sponsored links that you can click the 3 dots and dismiss. That's all I've noticed