add: if (!aFocusedWindow) return false;  
context: function isContentFrame(aFocusedWindow){  var focusedTop = Components.lookupMethod(aFocusedWindow, 'top')                             .call(aFocusedWindow);  return (focusedTop == window.content);}
meta: 10234	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/10234/05e0e83e11f753d1d8dc6a6b5266246e42e0da47/browser.js/clean/browser/base/content/browser.js