Mozilla has disabled Two Microsoft Corporation Plugins .NET Framework Assistant add-on and Windows Presentation Foundation for Firefox to stop a vulnerability that allows an attacker to take hold over Windows based machines.
According to Mozilla Blog
Because of the difficulties some users have had entirely removing the add-on, and because of the severity of the risk it represents if not disabled, we contacted Microsoft today to indicate that we were looking to disable the extension and plugin for all users via our block listing mechanism. Microsoft agreed with the plan, and we put the block list entry live immediately.
Update (Sunday Oct 18, 6:30pm PDT): Microsoft has now confirmed that the Framework Assistant add-on is not a vector for this attack, and we have removed the entry from the blocklist. We are also working on a mechanism to allow Firefox users to re-enable the WPF plugin ahead of its eventual removal from the blocklist. For more information, see Mike Shaver’s latest blog post.