diff --git a/SysBuild/xsys/firefox b/SysBuild/xsys/firefox
index a54abd7..12830d6 100644
--- a/SysBuild/xsys/firefox
+++ b/SysBuild/xsys/firefox
@@ -169,4 +169,6 @@ postmk() {
 	cd $TMPROOT/opt/firefox/include
 	ln -s firefox* firefox
 	cd -
+	echo "pref(\"network.protocol-handler.app.mailto\", \"thunderbird\");" \
+	     >>$TMPROOT/opt/firefox/lib/firefox/defaults/pref/firefox.js
 }