A friend just flagged to us the following activity on the Mozilla message boards indicating the latest updates to the browser code that is the base for the Firefox browser will dramatically change the handling of third party cookies. Comments by Dan Wittes, leader of the cookie module at Mozilla, explain that the current default ‘accept third party cookies’ option would result in changed behavior to make 3rd party cookies persistent only for the session. Unchecking the default would completely disable all third party cookies.
So if a user keeps their computer on and browser open, tracking across sites will continue, but if a user closes their browser, tracking cookies will be deleted. (Of course there are folks who track across sites but rely on various methods to ensure their cookies are relayed in fisrt party context – there may be some legitimate circumstances to do this, but we think that any company doing that in order to avoid browser controls is likely to come across the radar screen of the FTC before too long.)