Seems like in this particular case, notwithstanding Google's long history of abuse and opportunism, this one is a genuine case when you enable a cookie for technical and not (necessarily) user tracking reasons.
In other cases - yes. In this one - not necessarily. The content itself is hosted on a different domain and for a good reason. Most importantly though, they don't need another cookie to track you: you are already there, they know who you are, they know you already requested a resource, and they can track you over all Google services and many others anyway. They don't need to use a dirty trick to send you an additional cookie although the very fact that we're having the discussion means Firefox made a good decision and hopefully people will notice this pattern more.
