diff --git a/skin/classic/treestyletab/base-colors.css b/skin/classic/treestyletab/base-colors.css
index edcb8899..bea5d468 100644
--- a/skin/classic/treestyletab/base-colors.css
+++ b/skin/classic/treestyletab/base-colors.css
@@ -1,3 +1,5 @@
+@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
+
 :root[devtoolstheme="light"] {
 	--tabs-newtab-button-background:-moz-dialog;
 	--tabs-newtab-button-background-hover:ThreeDHighlight;