Problem with GetXULWindowEnumerator() API in firefox 3.5

Hi All,

I have small addon which has following snippet to enumerate
XULWindows.

=============
nsresult result;
nsCOMPtr<nsIWindowMediator> iWindowMediator = do_GetService
(NS_WINDOWMEDIATOR_CONTRACTID, &result);
result = iWindowMediator->GetXULWindowEnumerator(0, getter_AddRefs
(m_ienum));

nsCOMPtr<nsIXULWindow> webShellWindow(do_QueryInterface(pis,
&result));
=============

This is working fine with firefox 3.0 but with firefox 3.5, always
webShellWindow is returned null.
GetXULWindowEnumerator() API returns XULwindows, so I am not able to
understand why webShellWindow is returned null in firefox 3.5.

And also could you please help me with values I can pass for first
parameter in >GetXULWindowEnumerator(x,x) API.

Regards
Srinivas