Using JavaScript with OnContextMenu and event.shiftKey at the same time

Question

• I can use OnContextMenu() to detect right-clicks.
• I can use event.shiftKey to detect if the user is holding down the shift key.
• But I can't seem to get BOTH working at the same time, but only in FireFox. (Chrome works fine)
• FireFox just displays the normal right-click menu instead

https://jsfiddle.net/pf1grqx8/

<div style='border:1px solid black; width:555px;' oncontextmenu='return Test(event);'>
Ctrl + rightClick works <br><br>
Shift + rightClick does not work in Firefox (but works ok in Chrome)
</div>

and

function Test(e)
{
    alert(e.ctrlKey + "\n\n" +  e.shiftKey);
    return false;
}