--- a/dom/media/fmp4/MP4Decoder.cpp
+++ b/dom/media/fmp4/MP4Decoder.cpp
@@ -175,17 +175,17 @@ MP4Decoder::CanHandleMediaType(const nsA
 
   return CanHandleMediaType(NS_ConvertUTF16toUTF8(mimeType), codecs);
 }
 
 /* static */
 bool
 MP4Decoder::IsEnabled()
 {
-  return Preferences::GetBool("media.mp4.enabled");
+  return Preferences::GetBool("media.mp4.enabled", true);
 }
 
 // sTestH264ExtraData represents the content of the avcC atom found in
 // an AVC1 h264 video. It contains the H264 SPS and PPS NAL.
 // the structure of the avcC atom is as follow:
 // write(0x1);  // version, always 1
 // write(sps[0].data[1]); // profile
 // write(sps[0].data[2]); // compatibility

--- a/dom/media/platforms/PDMFactory.cpp
+++ b/dom/media/platforms/PDMFactory.cpp
@@ -77,41 +77,41 @@ PDMFactory::Init()
   MOZ_ASSERT(NS_IsMainThread());
   static bool alreadyInitialized = false;
   if (alreadyInitialized) {
     return;
   }
   alreadyInitialized = true;
 
   Preferences::AddBoolVarCache(&sUseBlankDecoder,
-                               "media.use-blank-decoder");
+                               "media.use-blank-decoder", false);
 #ifdef MOZ_GONK_MEDIACODEC
   Preferences::AddBoolVarCache(&sGonkDecoderEnabled,
-                               "media.gonk.enabled", false);
+                               "media.gonk.enabled", true);
 #endif
 #ifdef MOZ_WIDGET_ANDROID
   Preferences::AddBoolVarCache(&sAndroidMCDecoderEnabled,
                                "media.android-media-codec.enabled", false);
   Preferences::AddBoolVarCache(&sAndroidMCDecoderPreferred,
                                "media.android-media-codec.preferred", false);
 #endif
 
   Preferences::AddBoolVarCache(&sGMPDecoderEnabled,
-                               "media.gmp.decoder.enabled", false);
+                               "media.gmp.decoder.enabled", true);
 #ifdef MOZ_FFMPEG
   Preferences::AddBoolVarCache(&sFFmpegDecoderEnabled,
-                               "media.ffmpeg.enabled", false);
+                               "media.ffmpeg.enabled", true);
 #endif
 #ifdef MOZ_FFVPX
   Preferences::AddBoolVarCache(&sFFVPXDecoderEnabled,
-                               "media.ffvpx.enabled", false);
+                               "media.ffvpx.enabled", true);
 #endif
 #ifdef XP_WIN
   Preferences::AddBoolVarCache(&sWMFDecoderEnabled,
-                               "media.wmf.enabled", false);
+                               "media.wmf.enabled", true);
 #endif
 
   Preferences::AddBoolVarCache(&sEnableFuzzingWrapper,
                                "media.decoder.fuzzing.enabled", false);
   Preferences::AddUintVarCache(&sVideoOutputMinimumInterval_ms,
                                "media.decoder.fuzzing.video-output-minimum-interval-ms", 0);
   Preferences::AddBoolVarCache(&sDontDelayInputExhausted,
                                "media.decoder.fuzzing.dont-delay-inputexhausted", false);