--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -517,16 +517,17 @@ static const nsDefaultMimeTypeEntry defa
   { "application/xhtml+xml", "xhtml" },
   { "application/xhtml+xml", "xht" },
   { TEXT_PLAIN, "txt" },
   { VIDEO_OGG, "ogv" },
   { VIDEO_OGG, "ogg" },
   { APPLICATION_OGG, "ogg" },
   { AUDIO_OGG, "oga" },
   { AUDIO_OGG, "opus" },
+  { APPLICATION_PDF, "pdf" },
   { VIDEO_WEBM, "webm" },
   { AUDIO_WEBM, "webm" },
 #if defined(MOZ_WMF)
   { VIDEO_MP4, "mp4" },
   { AUDIO_MP4, "m4a" },
   { AUDIO_MP3, "mp3" },
 #endif
 #ifdef MOZ_RAW