Kind of an important point: this appears to be an attempt to make third party cookies useless, without actually disabling them since many sites depend on them.

Kind of an important point: this appears to be an attempt to make third party cookies useless, without actually disabling them since many sites depend on them. This is achieved in two ways: 1. By allowing third party cookies, but compartmentalizing them by the first-party site that sent the request (a much better name for this feature would be "per-site cookie containers", "total cookie protection" is completely uninformative). 2. By using a heuristic to selectively allow cookies to be accessed across the container boundary if they are actually needed, e.g. for logins. To answer your question, this doesn't make sense as "day one behavior" because it's basically a patch to work around a historical problem with as little breakage as possible. If you were setting up cookie permissions on day one, knowing what we know now, you wouldn't kneecap third party cookies, you'd disable them entirely. Mozilla is trying to make third party cookies useless for 99% of what they're used for: if that's how you feel about third party cookies, you'd just not implement them. Incidentally, I do block all third party cookies by default and have for years. That's a much stronger approach than the compartmentalization that Mozilla is attempting. I can count on one hand the number of sites I've seen break because of this, most of them are happy to let these cookies fail silently.