So that's what third party cookies are. What this does is make it so that when you go to example.com and you get a Google cookie, that cookie is only associated with example.com, and your random.org Google cookie will be specific to that site.

A site will be able to use Google to track how you use their site, which is a fine and valid thing, but they or Google don't get to see how you use a different site. (Google doesn't actually share specifics, but they can see stuff like "behavior on one site led to sale on the other")

We'll have to see what happens but what you are talking about is what Mozilla calls Third-Party Cookies and... they are aware of it.

https://support.mozilla.org/en-US/kb/third-party-cookies-firefox-tracking-protection#:~:text=Third%2Dparty%20cookies%20are%20cookies,considered%20a%20third%2Dparty%20cookie.

I can't entirely tell if that means they will be put in the facebook cookie jar or if it will be put in the TentaclePorn Dot Org (don't go there, it is probably a real site and probably horrifying) cookie jar. If the former? Then only facebook themselves have that which... is still a lot better I guess? If the latter then that is basically exactly what we all want but a lot of sites are gonna break (par for the course with Firefox but...).

It was - in the ancient times. Then, there were 3rd party cookies which you had to manually approve upon the initial creation. And then it went all down south and got abused via CDNs and ad networks.