There are different levels of "block third-party cookies".
Safari blocks _setting_ third-party cookies by default. It does not block _sending_ of third-party cookies, last I checked.
Firefox's third party cookie setting blocks setting and sending and doing that _definitely_ breaks some sites (e.g. some e-payment systems). Hence it not being enabled by default.
The problem is that the Safari approach is not good enough to prevent the tracking issue where Facebook is concerned: As long as you visited Facebook directly so that it got a chance to set a cookie, that cookie will get sent every time the Like button is fetched. So at that point Facebook knows who you are (via the cookie) and which site you visited (because the Like button itself has access to that information).
The Safari approach _does_ help deal with tracking via ad networks that you don't have any kind of first-party relationship with, since those would never get a chance to set a cookie to start with. So it's definitely a good idea, and I wish more browsers would adopt it. But it doesn't help with the Googles and Facebooks. Unless, of course, there were some sort of requirement that the "user-facing parts" and "ad parts" of those organizations use different domains. Would be pretty hard to get something like that written down properly, even.
Safari blocks _setting_ third-party cookies by default. It does not block _sending_ of third-party cookies, last I checked.
Firefox's third party cookie setting blocks setting and sending and doing that _definitely_ breaks some sites (e.g. some e-payment systems). Hence it not being enabled by default.
The problem is that the Safari approach is not good enough to prevent the tracking issue where Facebook is concerned: As long as you visited Facebook directly so that it got a chance to set a cookie, that cookie will get sent every time the Like button is fetched. So at that point Facebook knows who you are (via the cookie) and which site you visited (because the Like button itself has access to that information).
The Safari approach _does_ help deal with tracking via ad networks that you don't have any kind of first-party relationship with, since those would never get a chance to set a cookie to start with. So it's definitely a good idea, and I wish more browsers would adopt it. But it doesn't help with the Googles and Facebooks. Unless, of course, there were some sort of requirement that the "user-facing parts" and "ad parts" of those organizations use different domains. Would be pretty hard to get something like that written down properly, even.