This really isn't my area of expertise. Can you provide a link to what you're talking about? AFAIK, if you have third-party cookies set to "never", Facebook sees exactly the same (lack of) cookies no matter where I click on a like button outside of Facebook.com itself. So, if Firefox does what you say, I still don't see what it has to do with the third-party cookie option gcp pointed to.
Also, in trying to figure out exactly what feature you're talking about, I've come across quite a lot of sources that suggest that Safari has similar or more strict default settings than Firefox regarding third-party cookies.
It's part of the container tabs concept, but automatically putting each first-level origin (what you see in the URL bar) into its own container. This is atm only an about:config option in Nightly.
So how do you use this with systems that need it - can you log into the site for this "container"? Or would it leave the container when you go to the sign-in page? Like if I'm using Google or Facebook for single-sign-on, or I want to comment on a Disqus thread below a news article - since it's in its own container, I'm not logged in on this other site, but if I click a link to log in and it takes me away from this site, it takes me away from this container.
That's actually an interesting issue I had with it, in some versions that actually didn't work. But, somehow, now it does, and I don't know why (haven't read up on it yet).
Also, in trying to figure out exactly what feature you're talking about, I've come across quite a lot of sources that suggest that Safari has similar or more strict default settings than Firefox regarding third-party cookies.