diff --git a/toolkit/themes/osx/global/toolbar.css b/toolkit/themes/osx/global/toolbar.css
index 38614fb89857..820436f142c5 100644
--- a/toolkit/themes/osx/global/toolbar.css
+++ b/toolkit/themes/osx/global/toolbar.css
@@ -4,6 +4,12 @@
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 
+toolbox {
+  /* Setting -moz-appearance:toolbox causes sheets to attach under the
+   * toolbox and has no other effects. It doesn't render anything. */
+  -moz-appearance: toolbox;
+}
+
 toolbar {
   min-width: 1px;
   min-height: 20px;