Advertisement campaign a, takes people to landing site b.
Mozilla would be in charge of how long campaigns run, and what types of URLs would be allowed. The alternative is those ads get blocked and the advertiser gets nothing.
So yeah, the advertiser could tell that a Firefox user visited a given link, but they couldn't identify anything in particular about that user, other than their search history matched one of the criteria for the campaign.
That's honestly pretty acceptable. Other advertisers know the site the ad was served on, cookies from that site, potentially a nonce per user, etc. This method strips most of that, and only lets them know that it was a Firefox user during a given campaign. Mozilla could do audits to check if they're doing anything more to fingerprint users, and if so, drop the advertiser.
If those users would otherwise block ads, theoretically those advertisements are more valuable because those users are much harder to target. So advertisers may be willing to compromise here, since the alternative is no revenue. Mozilla would share revenue with sites, so there's an incentive for websites to opt-in as well.