diff --git a/toolkit/mozapps/extensions/content/extensions.xul b/toolkit/mozapps/extensions/content/extensions.xul
index c5eeb534ff..292ecf8d37 100644
--- a/toolkit/mozapps/extensions/content/extensions.xul
+++ b/toolkit/mozapps/extensions/content/extensions.xul
@@ -186,7 +186,7 @@
              placeholder="&search.placeholder;"/>
   </hbox>
 
-  <hbox flex="1">
+  <hbox id="main" flex="1">
 
     <!-- category list -->
     <richlistbox id="categories">