The Firefox add-on was silently added to Firefox when users downloaded a service pack for the Microsoft.NET Framework. Annoyances.org explains why this is a security problem: This update adds to Firefox one of the most dangerous vulnerabilities present in all versions of Internet Explorer. The extension is now enabling drive-by downloads in Firefox, according to a report [washingtonpost.com] by Brian Krebs [Krebs] The extension was added silently to Firefox by users downloading the Microsoft.NET Framework service pack.
Source: https://threatpost.com/microsoft-sneaks-firefox-add-without-user-knowledge-052909/72749/