--- a/netwerk/mime/nsMimeTypes.h
+++ b/netwerk/mime/nsMimeTypes.h
@@ -156,16 +156,17 @@
 #define TEXT_RDF                            "text/rdf"
 #define TEXT_XUL                            "application/vnd.mozilla.xul+xml"
 #define TEXT_ECMASCRIPT                     "text/ecmascript"
 #define TEXT_JAVASCRIPT                     "text/javascript"
 #define TEXT_XSL                            "text/xsl"
 #define TEXT_EVENT_STREAM                   "text/event-stream"
 
 #define VIDEO_MPEG                          "video/mpeg"
+#define VIDEO_MP4                           "video/mp4"
 #define VIDEO_RAW                           "video/x-raw-yuv"
 #define VIDEO_OGG                           "video/ogg"
 #define VIDEO_WEBM                          "video/webm"
 #define APPLICATION_OGG                     "application/ogg"
 
 /* x-uuencode-apple-single. QuickMail made me do this. */
 #define UUENCODE_APPLE_SINGLE               "x-uuencode-apple-single"
 

--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -435,16 +435,19 @@ static nsDefaultMimeTypeEntry defaultMim
 #ifdef MOZ_OPUS
   { AUDIO_OGG, "opus" },
 #endif
 #endif
 #ifdef MOZ_WEBM
   { VIDEO_WEBM, "webm" },
   { AUDIO_WEBM, "webm" },
 #endif
+#ifdef MOZ_GSTREAMER
+  { VIDEO_MP4, "mp4" },
+#endif
 #ifdef MOZ_RAW
   { VIDEO_RAW, "yuv" }
 #endif
 };
 
 /**
  * This is a small private struct used to help us initialize some
  * default mime types.
@@ -506,16 +509,17 @@ static nsExtraMimeTypeEntry extraMimeEnt
   { TEXT_CSS, "css", "Style Sheet" },
   { VIDEO_OGG, "ogv", "Ogg Video" },
   { VIDEO_OGG, "ogg", "Ogg Video" },
   { APPLICATION_OGG, "ogg", "Ogg Video"},
   { AUDIO_OGG, "oga", "Ogg Audio" },
   { AUDIO_OGG, "opus", "Opus Audio" },
   { VIDEO_WEBM, "webm", "Web Media Video" },
   { AUDIO_WEBM, "webm", "Web Media Audio" },
+  { VIDEO_MP4, "mp4", "MPEG-4 Video" },
   { VIDEO_RAW, "yuv", "Raw YUV Video" },
   { AUDIO_WAV, "wav", "Waveform Audio" }
 };
 
 #undef MAC_TYPE
 
 /**
  * File extensions for which decoding should be disabled.