diff --git a/platform/firefox/vapi-background.js b/platform/firefox/vapi-background.js
index 24e9da1..daa5a38 100644
--- a/platform/firefox/vapi-background.js
+++ b/platform/firefox/vapi-background.js
@@ -1716,6 +1716,7 @@ var httpObserver = {
         14: 'font',
         15: 'image',
         16: 'websocket',
+        20: 'xmlhttprequest',
         21: 'image'
     },
     mimeTypeMap: {