Firefox

They didn't really hide it, they just didn't advertise it. It was in the release notes, hence why the media picked up on it. And on release, there was a checkbox in the normal settings to opt-out, so it's honestly not that bad.

FakeSpot

That's an opt-in extension, it's not part of the core browser. I honestly don't know much about it, and their privacy policy isn't appealing, so I won't use it. If it becomes part of Firefox by default, I'll disable it.

In what universe are advertisers going to use this instead of, not in addition to, other telemetry?

What telemetry is this providing? AFAIK, Mozilla isn't providing any kind of personalized info, it's merely aggregated data.

And the reason they'd pick this is to get access to privacy-minded people who would otherwise block their ads, but may choose to exempt these ads. Mozilla has some anti-tracking features, and there's a significant subset of Firefox users that block ads out of principle of avoiding tracking. If websites want to get some of that advertising revenue, they'll comply. That benefits all Firefox users, because some sites may choose to use this method of targeted ads, which still provides the site with ad revenue without providing the advertisers with details on their customers.

That's the idea here. It's not going to happen on day 1, but having the capability means Mozilla can pilot it and see if websites are interested. And it's possible Mozilla's ads are more relevant because they have access to browsing history, not just whatever advertisers were able to figure out from their network of ads.