help with javascript - centering a modal dialog

I have this code that I use to center a modal dialog.    it used to work for IE and Firefox.  My harddrive crashed, and I had to reinstall my entire development environment.    Now, this code works for IE,  but not Firefox.  Any idea why?   Any better way to center a modal dialog?

       

var WIDTH = 850;
	var HEIGHT = 850;
		
	function ShowIncident(incident_id) {
 	
		var top = (screen.height / 2) - (HEIGHT / 2) ;
    	       var left = (screen.width / 2) - (WIDTH / 2);
								
		page = "../tds_incident/ShowTds_Incident.aspx?Tds_incident=" + incident_id
		mywindow = window.showModalDialog(page, null,
			"scroll:yes; resizable:no; help:no; " +
			"dialogTop:" + top + "px;" +
			"dialogLeft:" + left + "px; " +
			"dialogWidth:" + WIDTH + "px; " +
			"dialogHeight:" + HEIGHT + "px; ");
			
	}

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