diff --git a/mozilla/toolkit/content/finddialog.js b/mozilla/toolkit/content/finddialog.js
index e6bb166e625..d6ebadc9cd9 100644
--- a/mozilla/toolkit/content/finddialog.js
+++ b/mozilla/toolkit/content/finddialog.js
@@ -107,13 +107,13 @@ function onLoad()
 
   // get the find instance
   var arg0 = window.arguments[0];                                               
-  if (arg0 instanceof window.opener.nsFindInstData) {
+  // If the dialog was opened from window.find(),
+  // arg0 will be an instance of nsIWebBrowserFind
+  if (arg0 instanceof Components.interfaces.nsIWebBrowserFind) {
+    gFindInst = arg0;
+  } else {  
     gFindInstData = arg0;                                                       
     gFindInst = gFindInstData.webBrowserFind;                                   
-  } else {                                                                      
-    // If the dialog was opened from window.find(), findInst will be an         
-    // nsISupports interface, so QueryInterface anyway to nsIWebBrowserFind.    
-    gFindInst = arg0.QueryInterface(Components.interfaces.nsIWebBrowserFind);   
   }                                                                             
 
   fillDialog();