I mean people freaking out about this don't actually understand what's happening and why Mozilla is doing it. Mozilla is trying to build a new privacy-based advertising. The feature needs to be opt-in by default in order to have a chance to become mainstream. Forget about the technical details and whether the user understands what it is. Most people don't change default settings. So they can never get websites to try this better technology if their own users aren't adopting it.
I also hate the attitude of this community they think Firefox is built for them(ultra tech savy, extremely privacy concious) when 99% of their users are not these things. If you want ultra privacy, go use Libreawolf or whatever. Those solutions are for that type of person. Firefox and Mozilla builds for the average person, which is why they correctly say that the user won't understand the feature. (Anyone says otherwise is in a tech bubble and haven't seen normal people interacting with their computers).