Firefox

Perhaps. But they can also intuit that if someone gets to the landing page without clicking the link, because that's what that program appeals to. They also likely have a variety of other categories as well, such as non-obese and non-fit, lower class but living with parents, upper-class and single, etc.

But the important thing is that they wouldn't know which website the ad was served from or a unique identifier from the website to correlate to other data. If it's replacing Google or Facebook ads, that can be a lot of data, including my occupation, hobbies, accounts at other websites, etc. If all they get is that I was at least a >X% match for their ad-campaign through Mozilla, I'm fine with that. I can always clear/prune my browsing history to reset what types of categories I fall into.

But yeah, I get that many advertisers aren't going to be interested giving up that much data. However, if the alternative is no ads whatsoever, maybe that's attractive enough that they buy in. Idk, but that's my policy. If the ads won't respect my privacy, I'll block them. That's my line in the sand. If Mozilla offers a product I'm okay with, I'd be willing to disable my ad-blocker for those sites that opt-in.