It's not currently rolled out to any advertisers AFAIK, but when it does, I imagine it could become a significant revenue stream. So yes, hypothetical.

I don't think this feature is harmful in any way. If you block ads, it won't impact you. If you don't, you'll either get the current type of ads or the more private ads, so it either does nothing or improves your privacy a bit. It sounds like a win-win to me, which is rare in privacy circles, with the only caveat being a theoretical issue from Mozilla technically having access to your metadata, which they technically already do since you're using their browser.