The product development team for Windows Live Mail rolled back the M6 release of Windows Live Mail yesterday, this is all we know so far:

You may have noticed that some of the new features in Windows Live Mail Beta have gone missing! Well, you’re right. We hit a couple of bumps with M6, our most recent release, but we’re working hard (and very late on a Friday) to bring back the great new features. In the meantime, we’ve restored the service to the prior release so that you can access your mail quickly and reliably without hitting any of those bumps. This is all new stuff we’re doing so we appreciate your patience while we work the kinks out.
 
For all you Firefox users out there, this means that you’re getting our Classic Hotmail experience for the time being. Don’t worry we’ll get your new Windows Live experience working as soon as we can.  And if you’ve been enjoying your Russian language version of Windows Live Mail, we’ll bring that right back, too.

Via: Windows LIve Mail Team Blog