Make sure the sidebar has the extrachromme class on it for popups.

browser/base/content/browser.xul

@@ -1092,7 +1092,7 @@
   </toolbox>
   
   <hbox flex="1" id="browser">
-    <vbox id="sidebar-box" hidden="true">
+    <vbox id="sidebar-box" hidden="true" class="chromeclass-extrachrome">
       <sidebarheader align="center">
         <label id="sidebar-title" persist="value" flex="1" crop="end"/>
         <image id="sidebar-throbber"/>