This is from October 2009 (yes, more than a year ago), when Mozilla blocked two .NET-related plugins because of a security flaw (with Microsoft's blessing, no less). As far as I know, both plugins were both unblocked within a week or so. See http://shaver.off.net/diary/2009/10/19/update-on-the-net-fra... for more information on what happened.
Mozilla is automatically blocking two [Firefox] add-ons: the Microsoft .Net Framework Assistant and a related plugin called the Windows Presentation Foundation