"Total cookie protection" from Firefox

IIUC, this new feature is really only about third-party cookies, and means that even foo.com sets a cookie with path "/", if resources from foo.com are embedded in sites from different origins, then they will be different sets of cookies.

Certainly the feature as described doesn't *sound* any different from the status quo for first-party cookies. It's possible I'm missing something though because this seems strictly weaker than just blocking third party cookies altogether, which is absolutely a valid strategy if you're willing to permit a tiny number of exceptions, and from that perspective this half-way version seems kind of pointless.