My concern is that it relies on differential privacy, or privacy through deniability. Which seems like a poor fit when it comes down to submitting URLs visited, unless they plan on submitting fake URLs when the "coin flip" comes up as tails twice in a row?

Not to mention, people will tend to visit the same websites repeatedly. The entire premise of DP is that the real data will stand out from the noise, creating a compelling picture of what an individual visits on the web. How will that aggregate data be anonymized, when it is reported with (a minimum of) an IP?

In short, this still requires a lot of trust in Mozilla, even with the DP algorithm, to not do the wrong thing with the dataset. And, in my eyes, making this opt-out and not opt-in already compromises that trust.