Blocking third-party cookies breaks some websites, including some bank websites. Then again, Safari on iOS and OS X defaults to only allowing first-party cookies and websites must surely want their services to work on iOS..

Mozilla tried to adopt Safari's cookie policy for Firefox, but backed down when the ad industry turned up the heat:

https://blog.mozilla.org/privacy/2013/02/25/firefox-getting-...

http://www.computerworld.com/article/2495739/internet/ad-ind...