ClientX Failing On Firefox?

I'm trying to create a simple image gallery that has a floating image
that popups up when the user clicks on a thumbnail. So far so good.
Now, when I tried to align the popup image so it's near the cursor,
things get a little screwy: it works fine for Internet Explorer 6
(although always appears near the top for a long page) but completely
fails with Firefox (causing it to ignore the rest of the JavaScript).
Can someone help explain why? The code that's causing the problem
(which is used to calculate the x and y positions) is:

var posx = 0;
var posy = 0;
if (!e) var e = window.event;
if (e.pageX || e.pageY)
{
nc.style.left = e.pageX * 0.6;
nc.style.top = e.pageY * 0.5;
}
else if (e.clientX || e.clientY)
{
nc.style.left = (e.clientX + document.body.scrollLeft) * 0.6;
nc.style.top = (e.clientY + document.body.scrollTop) * 0.5;
}