diff --git a/mozilla/mail/base/content/msgMail3PaneWindow.js b/mozilla/mail/base/content/msgMail3PaneWindow.js
index 55e1c4c1d09..76a1ccc70ca 100644
--- a/mozilla/mail/base/content/msgMail3PaneWindow.js
+++ b/mozilla/mail/base/content/msgMail3PaneWindow.js
@@ -899,6 +899,8 @@ function OnUnloadMessenger()
   // FIX ME - later we will be able to use onload from the overlay
   OnUnloadMsgHeaderPane();
 
+  OnUnloadFolderPane();
+
   OnMailWindowUnload();
 }
 
@@ -1178,6 +1180,12 @@ function OnLoadFolderPane()
   folderTree.addEventListener("mousedown",TreeOnMouseDown,true);
 }
 
+function OnUnloadFolderPane()
+{
+  var folderTreeBuilder = GetFolderTree().builder.QueryInterface(Components.interfaces.nsIXULTreeBuilder);
+  folderTreeBuilder.removeObserver(folderObserver);
+}
+
 // builds prior to 12-08-2001 did not have the labels column
 // in the thread pane.  so if a user ran an old build, and then
 // upgraded, they get the new column, and this causes problems.