It mentions: "Some websites (e.g. Microsoft's Hotmail, MSN, and Windows Live Mail webmail) use third-party cookies for purposes that are not necessarily privacy concerns, and disabling third-party cookies may cause problems with those sites."
May cause? Not necessarily? Seriously? Anyone with third party cookies disabled ever had a problem with a web mail service?
Mozilla is willing to break html5 video for all intents and purposes because h264 isn't open enough (but this oddly helps Google) and allow third party cookies by default of reasons that make no sense (but this oddly helps Google) and it gets it's money from... Oh that's weird.
Have you read it? They're saying that some websites use them for purposes that are not privacy concerns. As long as Google doesn't do this - by which I mean, as long as they only use third-party cookies for tracking - they're irrelevant, because Mozilla would shut them down anyway.
It's only sleazy if you see it as an attack and not a simple explanation.
I'm guessing that MS is using the cookie for a purpose that it was not intended for. That's not to say it's inappropriate, but it's up to them to understand the risks and make sure the app works in both states.
It mentions: "Some websites (e.g. Microsoft's Hotmail, MSN, and Windows Live Mail webmail) use third-party cookies for purposes that are not necessarily privacy concerns, and disabling third-party cookies may cause problems with those sites."
That's probably the reason.