@mcc isn't this a different thing to the Chrome stuff that was a subject of controversy? tracking the user is bad, but Chrome's big idea was to spy on all your online activity to figure out your “interests” and then tell advertisers about them, whereas this Firefox feature is, I guess, purely to know if ads are effective? at least that's the intention…
@hikari Chrome "Ad privacy" (formerly "Ad sandbox", formerly "Ad labs") comprises three distinct features with individual toggles. You are referring to one of them ("Topics"). However, another one of the three, "Ad measurement", appears to be identical to the feature implemented by Firefox (and Safari, for that matter). Unlike Firefox (or Apple?), Google as far as I know/have tested will not turn any of these three features on without disclosing to the user and giving them a chance to opt out.