This is extremely easy to do in uMatrix. If you don't want to use the other features, just set a default policy of allow all, then block third party cookies. You can then selectively allow access to cookies on a site by site basis.
Good point. So the technology is there, and an "expert" UI.
It would be neat to see Mozilla create an intuitive UI for this to enable first party isolation by default, like a permissions model for other things. A modal popup "Unbury.me wants to grant access to google.com", for example.
