My point is not the way you label gathering information from your users but rather that it is about implementing something Google proposed.
If the mechanism works, fine, but why should I use Firefox over Chromium then? Opt-out data collection is in violation to my core beliefs and what I believed to be Mozilla's principles.
Collecting data without asking the user about it is - to me - in violation to the very definition of privacy and calling some way to anonymise data (who guarantees that the cryptographic approach to this is not obsolete in a few years?) "differential privacy" is at the very least dishonest.
So, I read that, and already see two problems. One - DP provides privacy by deniability. How does that apply to URLs (or even just domains)? For a domain to show up, I have to have visited it (unless Firefox will report back random domains).
Two - DP is only really private over a small data set per individual. If DP were enabled for even two days, you could get a very accurate picture of the sites I visit, since a majority of the domains reported would be necessarily be accurate values.
> I'm pretty sure that the idea is to report back random (existing) domains, yes.
Here's a concern that comes up from that implementation option: any outliers from the set of existing domains (which would likely simply be implemented as a list of strings) would immediately be able to be called out as a "True" value, while a single reporting of a domain could reliably called out as a "False" value. Unless, of course, you choose a randomization algorithm which exhibits a very strong clustering trait.
You could also limit reports to those domains which are in the whitelist, but that would voluntarily neuter the reporting; something they seem less-than-eager to do.
Ultimately, it will all come down to the implementation details, which are unlikely to be available until after the opt-in release, and auditable by a remarkably small number of people in the open source community.
For more information, see https://en.wikipedia.org/wiki/Differential_privacy for instance.