Finding Absolute Position

I got a script from brothercake which gets the absolute position for an element. Its pretty neat - recursively adding up offsets. I got it from the image transition scripts on his site.

However, this script uses offsetTop, offsetLeft and offsetParent, which dont seem to be supported by FireFox (or maybe I'm doing something wrong).

Here's brothercake's script:

getRealPosition = function()
{
this.pos = (arguments[1] == 'x') ? arguments[0].offsetLeft : arguments[0].offsetTop;
this.tmp = arguments[0].offsetParent;
while(this.tmp != null)
{
this.pos += (arguments[1] == 'x') ? this.tmp.offsetLeft : this.tmp.offsetTop;
this.tmp = this.tmp.offsetParent;
}
return this.pos;
}

It seems to work for Opera and IE - but not FF. Cant say for other browsers. I've come to rely on it for a part of my 'cross-browser' page. Any thoughts on what I can do?

What I can't figure out is why it seems to work fine on brothercake's site - even in FireFox. I'm supposing that the positioning script must be working because the images appear in the right spot.