Hi jonpeterv, as it was [https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plugins-in-firefox/ announced] already a while ago, Firefox would join other modern browsers like Chrome and Edge and cease support for NPAPI-based plugins like Java or Silverlight (with the sole exception of Adobe Flash Player for the time being). This is now in place starting with Firefox 52 and subsequent versions. [[Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work?]]

silverlight is not working after macOS Sierra https://discussions.apple.com/thread/7678372?start=0&tstart=0 and https://www.macrumors.com/2016/06/14/safari-macos-sierra-plugins-disabled-default/ Maybe scan the URL https://www.virustotal.com/#/home/url Please let us know if this solved your issue or if need further assistance.