I think a big part of the problem is that they didn’t show anyone a notification or an onboarding dialog or whatever about this feature, when it got introduced.
Right. Not only didn’t they notify anybody, but they took to Reddit to defend the decision not to notify anybody:
we consider modal consent dialogs to be a user-hostile distraction from better defaults, and do not believe such an experience would have been an improvement here.
Which is strange, because Mozilla has no problem with popups in general.
User-unique gets collected, and then the user-unique data sent to a remote server.
Only on the remote server will this data be aggregated, or so Mozilla says.