from http://www.seopher.com/articles/fixing_the_permission_denied_to_get_property_xulelement_selectedindex_error

If you’re ever doing some AJAX stuff with a text-box input it’s possible that your Firebug will throw a very unusual error:

Error: [Exception… “‘Permission denied to get property XULElement.selectedIndex’ when calling method:

The problem lies in Firefox specifically and it’s autocomplete functionality. To fix it all you need to do [...]