Click coordinates in Firefox toolbar button
===

I want to get the click coordinates from the toolbar icon in my Firefox extension.

The code I am using is given below:
`//Toolbar code`

    <?xml version="1.0"?>                 
    <overlay id="myext-toolbar-Overlay"
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
        
    <?xml-stylesheet href="chrome://myext/skin/myextToolbarButton.css" type="text/css"?>
    
    <script src="chrome://myext/content/myextExtension.js" type="application/x-javascript" />
    
    <toolbarpalette id="BrowserToolbarPalette">
        <toolbarbutton id="myext-button" class="toolbarbutton-1"
          label="Projectbook Web Clipper"
          tooltiptext="Clip Web Page"
          oncommand="myextExtension.showPopup(event);" >
        </toolbarbutton>
    </toolbarpalette>
    
    </overlay>

On clicking the toolbar icon a popup is displayed. I want to display the popup at the position where the toolbar icon was clicked.