nsISessionStore
this kind of domwindow object can be obtained from other nsidomwindow objects (like the sidebar window object) by applying the 'mainwindow' statement from working_with_windows_in_chrome_code.
... this outermost or root window can also be obtained from the list returned by nsiwindowmediator; see example #3 in working_with_windows_in_chrome_code.

XUL Questions and Answers - Archive of obsolete content
see also http://developer.mozilla.org/en/docs...in_chrome_code how can i include a .js document from inside a javascript document?