hm. see if this works.

toolkit/mozapps/downloads/content/pref-downloads.xul

@@ -63,7 +63,7 @@
     </radiogroup>
   </groupbox>
 
-#ifndef MOZ_THUNDERBIRD
+#ifdef MOZ_PHOENIX
     <groupbox align="start">
     <caption label="&downloadManagerWindow.label;"/>
     
@@ -156,7 +156,7 @@
                 id="editFileHandler" oncommand="editFileHandler();"/>
         <button label="&removeFileHandler.label;" accesskey="&removeFileHandler.accesskey;"              
                 id="removeFileHandler" oncommand="removeFileHandler();"/>
-#ifndef MOZ_THUNDERBIRD
+#ifdef MOZ_PHOENIX
         <spring flex="1"/>
         <button label="&plugins.label;" accesskey="&plugins.accesskey;"
                 id="plugins" oncommand="showPlugins();"/>