That's actually a pretty elegant solution to the issues that most of Firefox's userbase would encounter if all third-party cookies were blocked (e.g. not being

That's actually a pretty elegant solution to the issues that most of Firefox's userbase would encounter if all third-party cookies were blocked (e.g. not being able to log in with or share via their Twitter and Facebooks accounts). How often do real users, particularly the ones who are unaware that cross-site tracking is even going on, visit domains like doubleclick.net or googleads.g.doubleclick.net? For comparison, I block third-party cookies in Chrome and doubt that most users would be able to navigate the process of understanding when that breaks features on sites and then resolving the issue by selectively allowing the impacted domains. So, I think that's a positive feature in Firefox's (soon-to-be) implementation.