@cuchaz No, Firefox does NOT send the report back

Firefox creates a report based on what the website asks, but does not give the result to the website. Instead, FF encrypts the report and anonymously submits it using the Distributed Aggregation Protocol to an “aggregation service”.

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.