--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -102,21 +102,24 @@ MediaLoadInvalidURI=Invalid URI. Load of
 # LOCALIZATION NOTE: %1$S is the media resource's format/codec type (basically equivalent to the file type, e.g. MP4,AVI,WMV,MOV etc), %2$S is the URL of the media resource which failed to load.
 MediaLoadUnsupportedTypeAttribute=Specified "type" attribute of "%1$S" is not supported. Load of media resource %2$S failed.
 # LOCALIZATION NOTE: %1$S is the "media" attribute value of the <source> element. It is a media query. %2$S is the URL of the media resource which failed to load.
 MediaLoadSourceMediaNotMatched=Specified "media" attribute of "%1$S" does not match the environment. Load of media resource %2$S failed.
 # LOCALIZATION NOTE: %1$S is the MIME type HTTP header being sent by the web server, %2$S is the URL of the media resource which failed to load.
 MediaLoadUnsupportedMimeType=HTTP "Content-Type" of "%1$S" is not supported. Load of media resource %2$S failed.
 # LOCALIZATION NOTE: %S is the URL of the media resource which failed to load because of error in decoding.
 MediaLoadDecodeError=Media resource %S could not be decoded.
+MediaWidevineNoWMFNoSilverlight=Trying to play Widevine with no Windows Media Foundation (nor Silverlight fallback), see https://support.mozilla.org/kb/fix-video-audio-problems-firefox-windows
+# LOCALIZATION NOTE: %S is a comma-separated list of codecs (e.g. 'video/mp4, video/webm')
+MediaWMFNeeded=To play video formats %S, you need to install extra Microsoft software, see https://support.mozilla.org/kb/fix-video-audio-problems-firefox-windows
+# LOCALIZATION NOTE: %S is a comma-separated list of codecs (e.g. 'video/mp4, video/webm')
+MediaPlatformDecoderNotFound=The video on this page can't be played. Your system may not have the required video codecs for: %S
 # LOCALIZATION NOTE: %S is a comma-separated list of codecs (e.g. 'video/mp4, video/webm')
 MediaCannotPlayNoDecoders=Cannot play media. No decoders for requested formats: %S
 # LOCALIZATION NOTE: %S is a comma-separated list of codecs (e.g. 'video/mp4, video/webm')
-MediaPlatformDecoderNotFound=The video on this page can't be played. Your system may not have the required video codecs for: %S
-# LOCALIZATION NOTE: %S is a comma-separated list of codecs (e.g. 'video/mp4, video/webm')
 MediaNoDecoders=No decoders for some of the requested formats: %S
 # LOCALIZATION NOTE: Do not translate "MediaRecorder".
 MediaRecorderMultiTracksNotSupported=MediaRecorder does not support recording multiple tracks of the same type at this time.
 # LOCALIZATION NOTE: %S is the ID of the MediaStreamTrack passed to MediaStream.addTrack(). Do not translate "MediaStreamTrack" and "AudioChannel".
 MediaStreamAddTrackDifferentAudioChannel=MediaStreamTrack %S could not be added since it belongs to a different AudioChannel.
 # LOCALIZATION NOTE: Do not translate "MediaStream", "stop()" and "MediaStreamTrack"
 MediaStreamStopDeprecatedWarning=MediaStream.stop() is deprecated and will soon be removed. Use MediaStreamTrack.stop() instead.
 # LOCALIZATION NOTE: Do not translate "DOMException", "code" and "name"

--- a/dom/media/DecoderDoctorDiagnostics.cpp
+++ b/dom/media/DecoderDoctorDiagnostics.cpp
@@ -261,41 +261,42 @@ DispatchNotification(nsISupports* aSubje
     obs->NotifyObservers(aSubject, "decoder-doctor-notification", json.get());
   }
 }
 
 void
 DecoderDoctorDocumentWatcher::ReportAnalysis(
   dom::DecoderDoctorNotificationType aNotificationType,
   const char* aReportStringId,
-  const nsAString& aFormats)
+  const nsAString& aParams)
 {
   MOZ_ASSERT(NS_IsMainThread());
 
   if (!mDocument) {
     return;
   }
 
-  const char16_t* params[] = { aFormats.Data() };
+  // 'params' will only be forwarded for non-empty strings.
+  const char16_t* params[1] = { aParams.Data() };
   DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::ReportAnalysis() ReportToConsole - aMsg='%s' params[0]='%s'",
            this, mDocument, aReportStringId,
-           NS_ConvertUTF16toUTF8(params[0]).get());
+           aParams.IsEmpty() ? "<no params>" : NS_ConvertUTF16toUTF8(params[0]).get());
   nsContentUtils::ReportToConsole(nsIScriptError::warningFlag,
                                   NS_LITERAL_CSTRING("Media"),
                                   mDocument,
                                   nsContentUtils::eDOM_PROPERTIES,
                                   aReportStringId,
-                                  params,
-                                  ArrayLength(params));
+                                  aParams.IsEmpty() ? nullptr : params,
+                                  aParams.IsEmpty() ? 0 : 1);
 
   // 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);
+      mDocument->GetInnerWindow(), aNotificationType, aParams);
   }
 }
 
 enum SilverlightPresence {
   eNoSilverlight,
   eSilverlightDisabled,
   eSilverlightEnabled
 };
@@ -316,89 +317,143 @@ CheckSilverlight()
         return plugin->IsEnabled() ? eSilverlightEnabled : eSilverlightDisabled;
       }
     }
   }
 
   return eNoSilverlight;
 }
 
+static void AppendToStringList(nsAString& list, const nsAString& item)
+{
+  if (!list.IsEmpty()) {
+    list += NS_LITERAL_STRING(", ");
+  }
+  list += item;
+}
+
 void
 DecoderDoctorDocumentWatcher::SynthesizeAnalysis()
 {
   MOZ_ASSERT(NS_IsMainThread());
 
   bool canPlay = false;
+#if defined(XP_WIN)
+  bool WMFNeeded = false;
+#endif
 #if defined(MOZ_FFMPEG)
   bool FFMpegNeeded = false;
 #endif
+  nsAutoString playableFormats;
   nsAutoString unplayableFormats;
+  nsAutoString supportedKeySystems;
   nsAutoString unsupportedKeySystems;
+  DecoderDoctorDiagnostics::KeySystemIssue lastKeySystemIssue =
+    DecoderDoctorDiagnostics::eUnset;
 
   for (auto& diag : mDiagnosticsSequence) {
     switch (diag.mDecoderDoctorDiagnostics.Type()) {
     case DecoderDoctorDiagnostics::eFormatSupportCheck:
       if (diag.mDecoderDoctorDiagnostics.CanPlay()) {
         canPlay = true;
+        AppendToStringList(playableFormats,
+                           diag.mDecoderDoctorDiagnostics.Format());
       } else {
+#if defined(XP_WIN)
+        if (diag.mDecoderDoctorDiagnostics.DidWMFFailToLoad()) {
+          WMFNeeded = true;
+        }
+#endif
 #if defined(MOZ_FFMPEG)
         if (diag.mDecoderDoctorDiagnostics.DidFFmpegFailToLoad()) {
           FFMpegNeeded = true;
         }
 #endif
-        if (!unplayableFormats.IsEmpty()) {
-          unplayableFormats += NS_LITERAL_STRING(", ");
-        }
-        unplayableFormats += diag.mDecoderDoctorDiagnostics.Format();
+        AppendToStringList(unplayableFormats,
+                           diag.mDecoderDoctorDiagnostics.Format());
       }
       break;
     case DecoderDoctorDiagnostics::eMediaKeySystemAccessRequest:
-      if (!diag.mDecoderDoctorDiagnostics.IsKeySystemSupported()) {
-        if (!unsupportedKeySystems.IsEmpty()) {
-          unsupportedKeySystems += NS_LITERAL_STRING(", ");
+      if (diag.mDecoderDoctorDiagnostics.IsKeySystemSupported()) {
+        AppendToStringList(supportedKeySystems,
+                           diag.mDecoderDoctorDiagnostics.KeySystem());
+      } else {
+        AppendToStringList(unsupportedKeySystems,
+                           diag.mDecoderDoctorDiagnostics.KeySystem());
+        DecoderDoctorDiagnostics::KeySystemIssue issue =
+          diag.mDecoderDoctorDiagnostics.GetKeySystemIssue();
+        if (issue != DecoderDoctorDiagnostics::eUnset) {
+          lastKeySystemIssue = issue;
         }
-        unsupportedKeySystems += diag.mDecoderDoctorDiagnostics.KeySystem();
       }
       break;
     default:
       MOZ_ASSERT(diag.mDecoderDoctorDiagnostics.Type()
                    == DecoderDoctorDiagnostics::eFormatSupportCheck
                  || diag.mDecoderDoctorDiagnostics.Type()
                       == DecoderDoctorDiagnostics::eMediaKeySystemAccessRequest);
       break;
     }
   }
 
-  if (!canPlay) {
+  // Look at Key System issues first, as they may influence format checks.
+  if (!unsupportedKeySystems.IsEmpty() && supportedKeySystems.IsEmpty()) {
+    // No supported key systems!
+    switch (lastKeySystemIssue) {
+    case DecoderDoctorDiagnostics::eWidevineWithNoWMF:
+      if (CheckSilverlight() != eSilverlightEnabled) {
+        DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unsupported key systems: %s, widevine without WMF nor Silverlight",
+                 this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
+        ReportAnalysis(dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
+                       "MediaWidevineNoWMFNoSilverlight", NS_LITERAL_STRING(""));
+        return;
+      }
+      break;
+    default:
+      break;
+    }
+  }
+
+  if (!canPlay && !unplayableFormats.IsEmpty()) {
+#if defined(XP_WIN)
+    if (WMFNeeded) {
+      DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - formats: %s -> Cannot play media because WMF was not found",
+               this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
+      ReportAnalysis(dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
+                     "MediaWMFNeeded", unplayableFormats);
+      return;
+    }
+#endif
 #if defined(MOZ_FFMPEG)
     if (FFMpegNeeded) {
       DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unplayable formats: %s -> Cannot play media because platform decoder was not found",
                this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
       ReportAnalysis(dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
                      "MediaPlatformDecoderNotFound", unplayableFormats);
-    } else
+      return;
+    }
 #endif
-    {
-      DD_WARN("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - Cannot play media, unplayable formats: %s",
-              this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
-      ReportAnalysis(dom::DecoderDoctorNotificationType::Cannot_play,
-                     "MediaCannotPlayNoDecoders", unplayableFormats);
-    }
-  } else if (!unplayableFormats.IsEmpty()) {
+    DD_WARN("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - Cannot play media, unplayable formats: %s",
+            this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
+    ReportAnalysis(dom::DecoderDoctorNotificationType::Cannot_play,
+                   "MediaCannotPlayNoDecoders", unplayableFormats);
+    return;
+  }
+  if (!unplayableFormats.IsEmpty()) {
     DD_INFO("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - Can play media, but no decoders for some requested formats: %s",
             this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
     if (Preferences::GetBool("media.decoderdoctor.verbose", false)) {
       ReportAnalysis(
         dom::DecoderDoctorNotificationType::Can_play_but_some_missing_decoders,
         "MediaNoDecoders", unplayableFormats);
     }
-  } else {
-    DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - Can play media, decoders available for all requested formats",
-             this, mDocument);
+    return;
   }
+  DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - Can play media, decoders available for all requested formats",
+           this, mDocument);
 }
 
 void
 DecoderDoctorDocumentWatcher::AddDiagnostics(DecoderDoctorDiagnostics&& aDiagnostics,
                                              const char* aCallSite)
 {
   MOZ_ASSERT(NS_IsMainThread());