--- a/dom/media/DecoderDoctorDiagnostics.cpp
+++ b/dom/media/DecoderDoctorDiagnostics.cpp
@@ -354,48 +354,51 @@ static void AppendToStringList(nsAString
   list += item;
 }
 
 void
 DecoderDoctorDocumentWatcher::SynthesizeAnalysis()
 {
   MOZ_ASSERT(NS_IsMainThread());
 
+  nsAutoString playableFormats;
+  nsAutoString unplayableFormats;
+  // Subsets of unplayableFormats that require a specific platform decoder:
 #if defined(XP_WIN)
-  bool WMFNeeded = false;
+  nsAutoString formatsRequiringWMF;
 #endif
 #if defined(MOZ_FFMPEG)
-  bool FFMpegNeeded = false;
+  nsAutoString formatsRequiringFFMpeg;
 #endif
-  nsAutoString playableFormats;
-  nsAutoString unplayableFormats;
   nsAutoString supportedKeySystems;
   nsAutoString unsupportedKeySystems;
   DecoderDoctorDiagnostics::KeySystemIssue lastKeySystemIssue =
     DecoderDoctorDiagnostics::eUnset;
 
   for (const auto& diag : mDiagnosticsSequence) {
     switch (diag.mDecoderDoctorDiagnostics.Type()) {
       case DecoderDoctorDiagnostics::eFormatSupportCheck:
         if (diag.mDecoderDoctorDiagnostics.CanPlay()) {
           AppendToStringList(playableFormats,
                              diag.mDecoderDoctorDiagnostics.Format());
         } else {
+          AppendToStringList(unplayableFormats,
+                             diag.mDecoderDoctorDiagnostics.Format());
 #if defined(XP_WIN)
           if (diag.mDecoderDoctorDiagnostics.DidWMFFailToLoad()) {
-            WMFNeeded = true;
+            AppendToStringList(formatsRequiringWMF,
+                               diag.mDecoderDoctorDiagnostics.Format());
           }
 #endif
 #if defined(MOZ_FFMPEG)
           if (diag.mDecoderDoctorDiagnostics.DidFFmpegFailToLoad()) {
-            FFMpegNeeded = true;
+            AppendToStringList(formatsRequiringFFMpeg,
+                               diag.mDecoderDoctorDiagnostics.Format());
           }
 #endif
-          AppendToStringList(unplayableFormats,
-                             diag.mDecoderDoctorDiagnostics.Format());
         }
         break;
       case DecoderDoctorDiagnostics::eMediaKeySystemAccessRequest:
         if (diag.mDecoderDoctorDiagnostics.IsKeySystemSupported()) {
           AppendToStringList(supportedKeySystems,
                              diag.mDecoderDoctorDiagnostics.KeySystem());
         } else {
           AppendToStringList(unsupportedKeySystems,
@@ -433,32 +436,33 @@ DecoderDoctorDocumentWatcher::Synthesize
         break;
     }
   }
 
   // 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...
+      // 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 (WMFNeeded) {
-        DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - formats: %s -> Cannot play media because WMF was not found",
-                 this, mDocument, NS_ConvertUTF16toUTF8(unplayableFormats).get());
+      if (!formatsRequiringWMF.IsEmpty()) {
+        DD_DEBUG("DecoderDoctorDocumentWatcher[%p, doc=%p]::SynthesizeAnalysis() - unplayable formats: %s -> Cannot play media because WMF was not found",
+                 this, mDocument, NS_ConvertUTF16toUTF8(formatsRequiringWMF).get());
         ReportAnalysis(dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
-                       "MediaWMFNeeded", unplayableFormats);
+                       "MediaWMFNeeded", formatsRequiringWMF);
         return;
       }
 #endif
 #if defined(MOZ_FFMPEG)
-      if (FFMpegNeeded) {
+      if (!formatsRequiringFFMpeg.IsEmpty()) {
         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());
+                 this, mDocument, NS_ConvertUTF16toUTF8(formatsRequiringFFMpeg).get());
         ReportAnalysis(dom::DecoderDoctorNotificationType::Platform_decoder_not_found,
-                       "MediaPlatformDecoderNotFound", unplayableFormats);
+                       "MediaPlatformDecoderNotFound", formatsRequiringFFMpeg);
         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);
       return;