When I visit the "Check Your Plugins Page" here: https://www.mozilla.org/en-US/plugincheck/ It shows that that I Silverlight 5.1.404160, and shows it as "Up to Date." That version has been out of date since the release of Silverlight 5.1.40728.0 back on August 11, 2015. That update fixed a vulnerability that allowed remote code execution! Even that has been superseded by Silverlight 5.1.40905.0, released September 29, 2015. So, why does your "Check Your Plugins" page report that an old vulnerable version of Silverlight is up to date?