Centering popup windows in the middle of your browser window

The script below works in both Safari and IE, but while it will pop open a window in Firefox, it doesn't size it or center it in the middle of the browser window.

I need it in the center in the bowser window due to multiple screens and some other issues.

Again, it works fine in both Safari and IE 7/8 but not FireFox.

Thanks for any help you can provide!

<!--//**************************************
// Name: Center Popup Windows
// Description:Center popup windows in the middle of your browser.
// By: Lewis E. Moten III
//
//
// Inputs:None
//
// Returns:None
//
//Assumes:None
//
//Side Effects:None
//This code is copyrighted and has limited warranties.
//Please see http://www.Planet-Source-Code.com/xq/ASP/txtCodeId.2634/lngWId.2/qx/vb/scripts/ShowCode.htm
//for details.
//**************************************
 -->
 
function getbrowserwidth()
    {
    	if (navigator.userAgent.indexOf("MSIE") > 0)
			{
        		return(document.body.clientWidth);
        	}
        else
 
 
            {
            		return window.outerWidth;
            	}
        }
        function getbrowserheight()
 
 
            {
            	if (navigator.userAgent.indexOf("MSIE") > 0)
 
 
                	{
                		return(document.body.clientHeight);
                	} 
                else
 
 
                    {
                    		return(window.outerHeight);
                    	}
                }
                var popup = new Object()
                function CenterPopup(URL, width, height)
 
 
                    {
                    	// get center of browser window
                    	var X = getbrowserwidth() / 2
                    	var Y = getbrowserheight() / 2
                    	
                    	popup = window.open(URL, 'PopUp', 'scrollbars=yes ' + 'width=' + width + ' ' + 'height=' + height + ' ' + 'top=' + (window.screenTop + (Y - (height/2))) + ' ' + 'left=' + (window.screenLeft + (X - (width/2))) 
                    		)
                    	
                    	popup.focus()
                }

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