diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index d2e34fb4ff..e0dd68e275 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1349,6 +1349,7 @@ pref("network.protocol-handler.external.vbscript", false);
 pref("network.protocol-handler.external.javascript", false);
 pref("network.protocol-handler.external.data", false);
 pref("network.protocol-handler.external.ms-help", false);
+pref("network.protocol-handler.external.mk", false);
 pref("network.protocol-handler.external.res", false);
 pref("network.protocol-handler.external.shell", false);
 pref("network.protocol-handler.external.vnd.ms.radio", false);