diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 88839b013..a4300ed5c 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -494,7 +494,7 @@ pref("media.track.enabled", false);
 
 // Whether to enable MediaSource support.
 pref("media.mediasource.enabled", true);
-pref("media.mediasource.mp4.enabled", false);
+pref("media.mediasource.mp4.enabled", true);
 pref("media.mediasource.webm.enabled", true);
 pref("media.mediasource.webm.audio.enabled", true);