How can I access flash by accessibility in Firefox?

Hi, all,

     I have a flash in a web page, I want to access it through
accessibility.

     I tried

     var accService = Components.classes["@mozilla.org/
accessibilityService;
1"].createInstance(Components.interfaces.nsIAccessibleRetrieval);
    //doc is the dom document.
    var acc = accService.getAccessibleFor(doc);
    //the flash is at (100,100).
    acc.getChildAtPoint(100,100);

   but I got nothing, is it possible to access flash by accessibility
in XPCOM?

   Thank you very  much!