This is tried and works in all browsers:
function getMousePos(e) {
return {x:e.clientX,y:e.clientY};
}
Now you can use it in an event like this:
document.onmousemove=function(e) {
var mousecoords = getMousePos(e);
alert(mousecoords.x);alert(mousecoords.y);
};
NOTE: The above function will return the mouse co-ordinates relative to the viewport, which is not affected by scroll. If you want to get co-ordinates including scroll, then use the below function.
function getMousePos(e) {
return {
x: e.clientX + document.body.scrollLeft,
y: e.clientY + document.body.scrollTop
};
}