https://support.mozilla.org/en-US/kb/privacy-preserving-attribution

Mozilla is prototyping this feature in order to inform an emerging Web standard designed to help sites understand how their ads perform without collecting data about individual people.

Privacy-preserving attribution works as follows:

1. Websites that show you ads can ask Firefox to remember these ads. When this happens, Firefox stores an “impression” which contains a little bit of information about the ad, including a destination website.
2. If you visit the destination website and do something that the website considers to be important enough to count (a “conversion”), that website can ask Firefox to generate a report. The destination website specifies what ads it is interested in.
3. Firefox creates a report based on what the website asks, but does not give the result to the website. Instead, Firefox encrypts the report and anonymously submits it using the Distributed Aggregation Protocol (DAP) to an “aggregation service”.
4. Your results are combined with many similar reports by the aggregation service. The destination website periodically receives a summary of the reports. The summary includes noise that provides differential privacy.

Also:

1. Website sagt dem Browser: Ich zeige ad <x> mit Ziel <y>
2. Andere Webseite sagt dem Browser: Ich bin <y> und mich interessiert ob der Benutzer zuvor <x> gesehen hat
3. Der Browser meldet an einen Aggregationsservice (nicht der Webseite <y> direkt) dass dies der Fall war
4. Der Aggregationsservice gibt regelmäßig aggregierte Zusammenfassungen von Meldungen mit Rauschen-“Verfälschung”