Why do you mention Safari and IE but but Firefox?
Firefox and Chrome doing the right thing won't help a large majority of non-technical users of the internet.
Can't edit from my mobile HN client.
in case anyone is wondering.
Link with pictures if anyone can't find it.
So, depending on if you click on a facebook like button on A.com or B.com, facebook sees two entirely separate browsers.
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.
TL;DR: Can't tell you how, but it somehow works.