diff --git a/mailnews/base/content/newmailalert.xul b/mailnews/base/content/newmailalert.xul
index 2df37494f6..75e839f546 100644
--- a/mailnews/base/content/newmailalert.xul
+++ b/mailnews/base/content/newmailalert.xul
@@ -30,6 +30,6 @@
       </vbox>
     </hbox>
 
-    <toolbarbutton id="closeButton" top="0" right="0" onclick="closeAlert();"/>
+    <toolbarbutton id="closeButton" class="close-icon" top="0" right="0" onclick="closeAlert();"/>
   </stack>
 </window>