that was my favourite test case when i developed the add on. I'm on the road now, so I can't verify if they still do it. it might depend on you having a YouTube account linked to your Google account. The technique I'm referring to involves redirecting you across other domains, allowing them to set 1st party cookies, and then back. this happens in an instant and "feels" like a 3rd party cookie to the user, because he does not even realize this happened. the browser will treat it as a 1st party cookie however.