--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -119,16 +119,18 @@ 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')
+MediaUnsupportedBeforeWindowsVista=Video formats %S unsupported by Microsoft before Windows Vista
+# 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')
 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".

--- a/dom/media/DecoderDoctorDiagnostics.cpp
+++ b/dom/media/DecoderDoctorDiagnostics.cpp
@@ -14,27 +14,33 @@
 #include "nsIDocument.h"
 #include "nsIObserverService.h"
 #include "nsIScriptError.h"
 #include "nsITimer.h"
 #include "nsIWeakReference.h"
 #include "nsPluginHost.h"
 #include "VideoUtils.h"
 
+#if defined(XP_WIN)
+#include "mozilla/WindowsVersion.h"
+#endif
+
 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 {
 
 struct NotificationAndReportStringId
 {
+  // Notification type, handled by browser-media.js.
   dom::DecoderDoctorNotificationType mNotificationType;
+  // Console message id. Key in dom/locales/.../chrome/dom/dom.properties.
   const char* mReportStringId;
 };
 
 // Class that collects a sequence of diagnostics from the same document over a
 // small period of time, in order to provide a synthesized analysis.
 //
 // Referenced by the document through a nsINode property, mTimer, and
 // inter-task captures.
@@ -244,31 +250,35 @@ DecoderDoctorDocumentWatcher::EnsureTime
 }
 
 static const NotificationAndReportStringId sMediaWidevineNoWMFNoSilverlight =
   { dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
     "MediaWidevineNoWMFNoSilverlight" };
 static const NotificationAndReportStringId sMediaWMFNeeded =
   { dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
     "MediaWMFNeeded" };
+static const NotificationAndReportStringId sMediaUnsupportedBeforeWindowsVista =
+  { dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
+    "MediaUnsupportedBeforeWindowsVista" };
 static const NotificationAndReportStringId sMediaPlatformDecoderNotFound =
   { dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
     "MediaPlatformDecoderNotFound" };
 static const NotificationAndReportStringId sMediaCannotPlayNoDecoders =
   { dom::DecoderDoctorNotificationType::Cannot_play,
     "MediaCannotPlayNoDecoders" };
 static const NotificationAndReportStringId sMediaNoDecoders =
   { dom::DecoderDoctorNotificationType::Can_play_but_some_missing_decoders,
     "MediaNoDecoders" };
 
 static const NotificationAndReportStringId*
 sAllNotificationsAndReportStringIds[] =
 {
   &sMediaWidevineNoWMFNoSilverlight,
   &sMediaWMFNeeded,
+  &sMediaUnsupportedBeforeWindowsVista,
   &sMediaPlatformDecoderNotFound,
   &sMediaCannotPlayNoDecoders,
   &sMediaNoDecoders
 };
 
 static void
 DispatchNotification(nsISupports* aSubject,
                      const NotificationAndReportStringId& aNotification,
@@ -537,19 +547,25 @@ DecoderDoctorDocumentWatcher::Synthesize
   // Next, check playability of requested formats.
   if (!unplayableFormats.IsEmpty()) {
     // Some requested formats cannot be played.
     if (playableFormats.IsEmpty()) {
       // No requested formats can be played. See if we can help the user, by
       // going through expected decoders from most to least desirable.
 #if defined(XP_WIN)
       if (!formatsRequiringWMF.IsEmpty()) {
-        DD_INFO("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unplayable formats: %s -> Cannot play media because WMF was not found",
-                this, mDocument, NS_ConvertUTF16toUTF8(formatsRequiringWMF).get());
-        ReportAnalysis(sMediaWMFNeeded, false, formatsRequiringWMF);
+        if (IsVistaOrLater()) {
+          DD_INFO("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unplayable formats: %s -> Cannot play media because WMF was not found",
+                  this, mDocument, NS_ConvertUTF16toUTF8(formatsRequiringWMF).get());
+          ReportAnalysis(sMediaWMFNeeded, false, formatsRequiringWMF);
+        } else {
+          DD_INFO("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unplayable formats: %s -> Cannot play media before Windows Vista",
+                  this, mDocument, NS_ConvertUTF16toUTF8(formatsRequiringWMF).get());
+          ReportAnalysis(sMediaUnsupportedBeforeWindowsVista, false, formatsRequiringWMF);
+        }
         return;
       }
 #endif
 #if defined(MOZ_FFMPEG)
       if (!formatsRequiringFFMpeg.IsEmpty()) {
         DD_INFO("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unplayable formats: %s -> Cannot play media because platform decoder was not found",
                 this, mDocument, NS_ConvertUTF16toUTF8(formatsRequiringFFMpeg).get());
         ReportAnalysis(sMediaPlatformDecoderNotFound,