ok - just for future reference in case anyone else runs into this issue
the error occurs when the javascript function invoking the PageMethod is
called from a different window
( mozilla seems to be maybe getting confused as to which objects live in
which window ? - or not  )

<script type="text/javascript">

   var imagebrowser =null;
   function SelectImage()
   {
       . . .
       imagebrowser =
window.open("Selector.aspx","Selector","menubar=0,minimizable=0,modal=1,status=0,toolbar=0,width=600,height=600");
   }

   /*

       PageMethods call will fail under mozilla if called from this
function when called from another window :  opener.SelectCallback(
selectdURL );
       splitting this into 2 functions and using timer to invoke second
part , letting this & functions complete solves the problem

       function SelectCallback( url )
       {
           imagebrowser.close();
           imagebrowser=null;
           . . .
           PageMethods.DoSomething( url , DoSomethingCallback , Timeout ,
Error );
       }

   */

   function SelectCallback( url )
   {
       imagebrowser.close();
       imagebrowser=null;
       setTimeout("SelectCallback2('"+url+"')",0);
   }

   function SelectCallback2( path )
   {
       . . .
       PageMethods.DoSomething( path , DoSomethingCallback, Timeout ,
Error );
   }

   function DoSomethingCallback( rslt )
   {
       . . .
   }

   function Timeout( txt )
   {
       alert('Timeout '+txt);
   }

   function Error( txt )
   {
       alert('Error '+txt);
   }

</script>

<input type="button" value="Select Image" onclick="GetImage();return 0;" />