[FF] Error when using undo

[Damian]

1. Clear browser cache/cookies etc.
   
2. Open Ajax sample on nightly build
   
3. Add some content
   
4. At end of the document add a horizontal line
   
5. Attempt to undo
   

Undo doesn't work on the horizontal line and the following error occurs:

l is null
[Break on this error] return h;},clone:function(h,i){var j=thi...h){var j=this;if(h.equals(j))return j;\r\n

This only happens on the first load of the editor.