rem: return aCommand == "cmd_cleanUp"; 
add: var commandNode = document.getElementById(aCommand); return commandNode && commandNode.parentNode == document.getElementById("downloadsCommands"); 
context:   supportsCommand: function (aCommand)  {    return aCommand == "cmd_cleanUp";  },
meta: 10520	/local1/tlutelli/issta_data/temp/all_js/javascript/2007_temp/2007/10520/a1584e4fdb4e43cf0a4e6b90d9625d12ca4aa99a/downloads.js/clean/toolkit/mozapps/downloads/content/downloads.js