Many third-party services (not just ads and tracking) currently rely on this behavior. That's not trivial to retract.
In the context of a browser, how much does anything else matter if you have installed some particular programming font?
if you've never seen EFF's Panopticlick check out https://panopticlick.eff.org
Code for all platforms available on github.
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.
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.
Link with pictures if anyone can't find it.
Yes, this would make single-sign-on harder, but it would make it explicit and be worth the trouble so that when the user is talking to A, they're not being tracked by A's friends B, C, and D.
Of course, the big problem: the best browser is owned by the advertiser who stands to lose under such an arrangement. So at best you'd need Safari or IE to spearhead such a change. You can ape it with browser extensions, but without a big browser maker pushing for this kind of shift some sites would just break under such a model (particularly single-sign-on services like Gmail and Facebook).