--- a/dom/media/DecoderDoctorDiagnostics.cpp
+++ b/dom/media/DecoderDoctorDiagnostics.cpp
@@ -8,16 +8,17 @@
 
 #include "mozilla/dom/DecoderDoctorNotificationBinding.h"
 #include "mozilla/Logging.h"
 #include "nsGkAtoms.h"
 #include "nsIDocument.h"
 #include "nsIObserverService.h"
 #include "nsITimer.h"
 #include "nsIWeakReference.h"
+#include "nsPluginHost.h"
 
 static mozilla::LazyLogModule sDecoderDoctorLog("DecoderDoctor");
 #define DD_LOG(level, arg, ...) MOZ_LOG(sDecoderDoctorLog, level, (arg, ##__VA_ARGS__))
 #define DD_DEBUG(arg, ...) DD_LOG(mozilla::LogLevel::Debug, arg, ##__VA_ARGS__)
 #define DD_INFO(arg, ...) DD_LOG(mozilla::LogLevel::Info, arg, ##__VA_ARGS__)
 #define DD_WARN(arg, ...) DD_LOG(mozilla::LogLevel::Warning, arg, ##__VA_ARGS__)
 
 namespace mozilla
@@ -288,16 +289,43 @@ DecoderDoctorDocumentWatcher::ReportAnal
   // For now, disable all front-end notifications by default.
   // TODO: Future bugs will use finer-grained filtering instead.
   if (Preferences::GetBool("media.decoderdoctor.enable-notification-bar", false)) {
     DispatchNotification(
       mDocument->GetInnerWindow(), aNotificationType, aFormats);
   }
 }
 
+enum SilverlightPresence {
+  eNoSilverlight,
+  eSilverlightDisabled,
+  eSilverlightEnabled
+};
+static SilverlightPresence
+CheckSilverlight()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  RefPtr<nsPluginHost> pluginHost = nsPluginHost::GetInst();
+  if (!pluginHost) {
+    return eNoSilverlight;
+  }
+  nsTArray<nsCOMPtr<nsIInternalPluginTag>> plugins;
+  pluginHost->GetPlugins(plugins, /*aIncludeDisabled*/ true);
+  for (const auto& plugin : plugins) {
+    for (const auto& mime : plugin->MimeTypes()) {
+      if (mime.LowerCaseEqualsLiteral("application/x-silverlight")
+          || mime.LowerCaseEqualsLiteral("application/x-silverlight-2")) {
+        return plugin->IsEnabled() ? eSilverlightEnabled : eSilverlightDisabled;
+      }
+    }
+  }
+
+  return eNoSilverlight;
+}
+
 void
 DecoderDoctorDocumentWatcher::SynthesizeAnalysis()
 {
   MOZ_ASSERT(NS_IsMainThread());
 
   bool canPlay = false;
 #if defined(MOZ_FFMPEG)
   bool FFMpegNeeded = false;