The New Firefox Cookie Policy
Found on Webpolicy on Saturday, 23 February 2013
The default Firefox cookie policy will, beginning with release 22, more closely reflect user privacy preferences.
Content from a third-party origin only has cookie permissions if its origin already has at least one cookie set.
The patch will spend about 6 weeks each in the pre-alpha, alpha, and beta builds.
It's good that they make this change; but it's still a bit surprising that it took them this long to decide something so obvious. Blocking third party cookies is one of the default changes to every Firefox installation.