--- a/dom/media/DecoderTraits.cpp
+++ b/dom/media/DecoderTraits.cpp
@@ -108,23 +108,16 @@ DecoderTraits::IsMP4SupportedType(const 
 #ifdef MOZ_FMP4
   return MP4Decoder::IsSupportedType(aType, aDiagnostics);
 #else
   return false;
 #endif
 }
 
 static bool
-IsMP3SupportedType(const nsACString& aType,
-                   const nsAString& aCodecs = EmptyString())
-{
-  return MP3Decoder::CanHandleMediaType(aType, aCodecs);
-}
-
-static bool
 IsAACSupportedType(const nsACString& aType,
                    const nsAString& aCodecs = EmptyString())
 {
   return ADTSDecoder::CanHandleMediaType(aType, aCodecs);
 }
 
 static bool
 IsWaveSupportedType(const nsACString& aType,
@@ -189,18 +182,17 @@ CanHandleCodecsType(const MediaContentTy
       return CANPLAY_YES;
     } else {
       // We can only reach this position if a particular codec was requested,
       // fmp4 is supported and working: the codec must be invalid.
       return CANPLAY_NO;
     }
   }
 #endif
-  if (IsMP3SupportedType(mimeType.Type().AsString(),
-                         aType.ExtendedType().Codecs().AsString())) {
+  if (MP3Decoder::IsSupportedType(aType)) {
     return CANPLAY_YES;
   }
   if (IsAACSupportedType(mimeType.Type().AsString(),
                          aType.ExtendedType().Codecs().AsString())) {
     return CANPLAY_YES;
   }
   if (IsFlacSupportedType(mimeType.Type().AsString(),
                           aType.ExtendedType().Codecs().AsString())) {
@@ -273,17 +265,17 @@ CanHandleMediaType(const MediaContentTyp
     return CANPLAY_MAYBE;
   }
 #endif
 #if !defined(MOZ_OMX_WEBM_DECODER)
   if (WebMDecoder::IsSupportedType(mimeType)) {
     return CANPLAY_MAYBE;
   }
 #endif
-  if (IsMP3SupportedType(mimeType.Type().AsString())) {
+  if (MP3Decoder::IsSupportedType(mimeType)) {
     return CANPLAY_MAYBE;
   }
   if (IsAACSupportedType(mimeType.Type().AsString())) {
     return CANPLAY_MAYBE;
   }
   if (IsFlacSupportedType(mimeType.Type().AsString())) {
     return CANPLAY_MAYBE;
   }
@@ -352,17 +344,17 @@ InstantiateDecoder(const MediaContentTyp
   RefPtr<MediaDecoder> decoder;
 
 #ifdef MOZ_FMP4
   if (MP4Decoder::IsSupportedType(aType, aDiagnostics)) {
     decoder = new MP4Decoder(aOwner);
     return decoder.forget();
   }
 #endif
-  if (IsMP3SupportedType(aType.Type().AsString())) {
+  if (MP3Decoder::IsSupportedType(aType)) {
     decoder = new MP3Decoder(aOwner);
     return decoder.forget();
   }
   if (IsAACSupportedType(aType.Type().AsString())) {
     decoder = new ADTSDecoder(aOwner);
     return decoder.forget();
   }
   if (OggDecoder::IsSupportedType(aType)) {
@@ -437,17 +429,17 @@ MediaDecoderReader* DecoderTraits::Creat
   }
 
 #ifdef MOZ_FMP4
   if (MP4Decoder::IsSupportedType(*type,
                                   /* DecoderDoctorDiagnostics* */ nullptr)) {
     decoderReader = new MediaFormatReader(aDecoder, new MP4Demuxer(aDecoder->GetResource()));
   } else
 #endif
-  if (IsMP3SupportedType(aType)) {
+  if (MP3Decoder::IsSupportedType(*type)) {
     decoderReader = new MediaFormatReader(aDecoder, new mp3::MP3Demuxer(aDecoder->GetResource()));
   } else
   if (IsAACSupportedType(aType)) {
     decoderReader = new MediaFormatReader(aDecoder, new ADTSDemuxer(aDecoder->GetResource()));
   } else
   if (IsWaveSupportedType(aType)) {
     decoderReader = new MediaFormatReader(aDecoder, new WAVDemuxer(aDecoder->GetResource()));
   } else
@@ -497,17 +489,17 @@ bool DecoderTraits::IsSupportedInVideoDo
     OggDecoder::IsSupportedType(*type) ||
     WebMDecoder::IsSupportedType(*type) ||
 #ifdef MOZ_ANDROID_OMX
     (MediaDecoder::IsAndroidMediaPluginEnabled() && IsAndroidMediaType(aType)) ||
 #endif
 #ifdef MOZ_FMP4
     MP4Decoder::IsSupportedType(*type, /* DecoderDoctorDiagnostics* */ nullptr) ||
 #endif
-    IsMP3SupportedType(aType) ||
+    MP3Decoder::IsSupportedType(*type) ||
     IsAACSupportedType(aType) ||
     IsFlacSupportedType(aType) ||
 #ifdef MOZ_DIRECTSHOW
     IsDirectShowSupportedType(aType) ||
 #endif
     false;
 }
 

--- a/dom/media/MP3Decoder.cpp
+++ b/dom/media/MP3Decoder.cpp
@@ -1,16 +1,17 @@
 
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "MP3Decoder.h"
+#include "MediaContentType.h"
 #include "MediaDecoderStateMachine.h"
 #include "MediaFormatReader.h"
 #include "MP3Demuxer.h"
 #include "PDMFactory.h"
 
 namespace mozilla {
 
 MediaDecoder*
@@ -32,19 +33,21 @@ MP3Decoder::CreateStateMachine() {
 bool
 MP3Decoder::IsEnabled() {
   RefPtr<PDMFactory> platform = new PDMFactory();
   return platform->SupportsMimeType(NS_LITERAL_CSTRING("audio/mpeg"),
                                     /* DecoderDoctorDiagnostics* */ nullptr);
 }
 
 /* static */
-bool MP3Decoder::CanHandleMediaType(const nsACString& aType,
-                                    const nsAString& aCodecs)
+bool MP3Decoder::IsSupportedType(const MediaContentType& aContentType)
 {
-  if (aType.EqualsASCII("audio/mp3") || aType.EqualsASCII("audio/mpeg")) {
-    return IsEnabled() &&
-      (aCodecs.IsEmpty() || aCodecs.EqualsASCII("mp3"));
+  if (aContentType.Type() == MEDIAMIMETYPE("audio/mp3")
+      || aContentType.Type() == MEDIAMIMETYPE("audio/mpeg")) {
+    return
+      IsEnabled()
+      && (aContentType.ExtendedType().Codecs().IsEmpty()
+          || aContentType.ExtendedType().Codecs().AsString().EqualsASCII("mp3"));
   }
   return false;
 }
 
 } // namespace mozilla

--- a/dom/media/MP3Decoder.h
+++ b/dom/media/MP3Decoder.h
@@ -5,25 +5,27 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #ifndef MP3Decoder_h_
 #define MP3Decoder_h_
 
 #include "MediaDecoder.h"
 
 namespace mozilla {
 
-class MP3Decoder : public MediaDecoder {
+class MediaContentType;
+
+class MP3Decoder : public MediaDecoder
+{
 public:
   // MediaDecoder interface.
   explicit MP3Decoder(MediaDecoderOwner* aOwner) : MediaDecoder(aOwner) {}
   MediaDecoder* Clone(MediaDecoderOwner* aOwner) override;
   MediaDecoderStateMachine* CreateStateMachine() override;
 
   // Returns true if the MP3 backend is preffed on, and we're running on a
   // platform that is likely to have decoders for the format.
   static bool IsEnabled();
-  static bool CanHandleMediaType(const nsACString& aType,
-                                 const nsAString& aCodecs);
+  static bool IsSupportedType(const MediaContentType& aContentType);
 };
 
 } // namespace mozilla
 
 #endif