A major difference is that the data is stored in your browser, and aggregated anonymously by Mozilla (also using differential privacy). Using the techniques you refer to, the ad platforms both store the data and then aggregate it, possibly promising to add differential privacy. The advantages I see are: (1) you can verify which data is collected by the browser and when/how it is sent to Mozilla, because this code is open source and running on your machine; (2) you maybe trust Mozilla more than an ad company.
Better for whom? Even more of normal internet operations flowing through ISRG is concerning itself. Let's Encrypt alone already gives them more power than any private organization should have.
