It's not hard to imagine how this concept could expand to the web at large. Rather than deciding what ad to display by consulting a user profile built on cross-site history tracking, a site could simply ask the browser for advice on what to show, without the site having to learn anything in particular about the user.

Good for the site because they get better advice; good for the user because they get more privacy and control.

(I've nothing to do with the team behind this at Mozilla, and have no idea what their roadmap actually is. But it's pretty clear that this is just a first step in a broader version of re-inventing advertising on the web, not a stand-alone attempt to generate a bit more revenue).