--- a/dom/media/mediasource/MediaSource.cpp
+++ b/dom/media/mediasource/MediaSource.cpp
@@ -58,48 +58,53 @@ PRLogModuleInfo* GetMediaSourceAPILog()
 #endif
 
 // Arbitrary limit.
 static const unsigned int MAX_SOURCE_BUFFERS = 16;
 
 namespace mozilla {
 
 static const char* const gMediaSourceTypes[6] = {
+// XXX: Disabled other temporarily on desktop to allow WebM testing.  For now,
+// set the developer-only media.mediasource.ignore_codecs pref to true to test
+// other codecs, and expect things to be broken.
+//
+// Disabled WebM in favour of MP4 on Firefox OS.
+#ifdef MOZ_GONK_MEDIACODEC
   "video/mp4",
   "audio/mp4",
+#else
   "video/webm",
   "audio/webm",
+#endif
+#if 0
+  "audio/mpeg",
+#endif
   nullptr
 };
 
 static nsresult
 IsTypeSupported(const nsAString& aType)
 {
   if (aType.IsEmpty()) {
     return NS_ERROR_DOM_INVALID_ACCESS_ERR;
   }
+  if (Preferences::GetBool("media.mediasource.ignore_codecs", false)) {
+    return NS_OK;
+  }
+  // TODO: Further restrict this to formats in the spec.
   nsContentTypeParser parser(aType);
   nsAutoString mimeType;
   nsresult rv = parser.GetType(mimeType);
   if (NS_FAILED(rv)) {
     return NS_ERROR_DOM_NOT_SUPPORTED_ERR;
   }
   bool found = false;
   for (uint32_t i = 0; gMediaSourceTypes[i]; ++i) {
     if (mimeType.EqualsASCII(gMediaSourceTypes[i])) {
-      if ((mimeType.EqualsASCII("video/mp4") ||
-           mimeType.EqualsASCII("audio/mp4")) &&
-          !Preferences::GetBool("media.mediasource.mp4.enabled", false)) {
-        break;
-      }
-      if ((mimeType.EqualsASCII("video/webm") ||
-           mimeType.EqualsASCII("audio/webm")) &&
-          !Preferences::GetBool("media.mediasource.webm.enabled", false)) {
-        break;
-      }
       found = true;
       break;
     }
   }
   if (!found) {
     return NS_ERROR_DOM_NOT_SUPPORTED_ERR;
   }
   // Check aType against HTMLMediaElement list of MIME types.  Since we've

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -408,24 +408,16 @@ pref("media.track.enabled", false);
 
 // Whether to enable MediaSource support
 #ifdef RELEASE_BUILD
 pref("media.mediasource.enabled", false);
 #else
 pref("media.mediasource.enabled", true);
 #endif
 
-#ifdef MOZ_WIDGET_GONK
-pref("media.mediasource.mp4.enabled", false);
-pref("media.mediasource.webm.enabled", false);
-#else
-pref("media.mediasource.mp4.enabled", false);
-pref("media.mediasource.webm.enabled", true);
-#endif
-
 #ifdef MOZ_WEBSPEECH
 pref("media.webspeech.recognition.enable", false);
 pref("media.webspeech.synth.enabled", false);
 #endif
 #ifdef MOZ_WEBM_ENCODER
 pref("media.encoder.webm.enabled", true);
 #endif
 #ifdef MOZ_OMX_ENCODER