Firefox users on Windows probably have the .NET Framework Assistant extension installed, even if they don't remember asking it to install (they didn't). Now Mozilla is blocking the extension for all users to stop a security breach.
It is, yet again, a good lesson in open-source security. Microsoft discovered a vulnerability in Internet Explorer and .NET-connected browsers that allowed a site with malicious code to, well, "own" your browser and install some other terrible stuff.
Microsoft issued a high-priority security patch for Windows systems and through Internet Explorer's update mechanism, but for Firefox users who haven't applied the patch, Mozilla has added the Microsoft .Net Framework Assistant and Windows Presentation Foundation extensions to its blocklist, noting that users should see the extensions disabled upon their next log-in.
If you still see those extensions enabled on your Windows system, Mozilla's security chief has written about the special means of removing them, as they often can't be disabled by default. Better still, if you see extensions in your Firefox Add-Ons menu that you can't quite remember installing, or question what purpose they serve, take this as a lesson in why uninstalling them might be a good idea.
.NET Framework Assistant Blocked to Disarm Security Vulnerability [Mozilla Security Blog via Yahoo News]