diff --git a/toolkit/content/charsetOverlay.js b/toolkit/content/charsetOverlay.js
index 7dcbabf362c4..b22d083cf742 100644
--- a/toolkit/content/charsetOverlay.js
+++ b/toolkit/content/charsetOverlay.js
@@ -255,7 +255,7 @@ function mailCharsetLoadListener (event)
     }
 }
 
-var wintype = document.firstChild.getAttribute('windowtype');
+var wintype = document.documentElement.getAttribute('windowtype');
 if (window && (wintype == "navigator:browser"))
 {
     var contentArea = window.document.getElementById("appcontent");
diff --git a/toolkit/content/contentAreaUtils.js b/toolkit/content/contentAreaUtils.js
index 7b4e076108f9..e689e9b0a186 100644
--- a/toolkit/content/contentAreaUtils.js
+++ b/toolkit/content/contentAreaUtils.js
@@ -72,7 +72,7 @@ function openNewTabWith(href, sourceURL, postData, event)
 
   // As in openNewWindowWith(), we want to pass the charset of the
   // current document over to a new tab. 
-  var wintype = document.firstChild.getAttribute('windowtype');
+  var wintype = document.documentElement.getAttribute('windowtype');
   var originCharset;
   if (wintype == "navigator:browser")
     originCharset = window.content.document.characterSet;
@@ -94,7 +94,7 @@ function openNewWindowWith(href, sourceURL, postData)
   // set, then extract the current charset menu setting from the current document and use it to
   // initialize the new browser window...
   var charsetArg = null;
-  var wintype = document.firstChild.getAttribute('windowtype');
+  var wintype = document.documentElement.getAttribute('windowtype');
   if (wintype == "navigator:browser")
     charsetArg = "charset=" + window.content.document.characterSet;