It seems the border is not taken into account in the JS offsetTop/Left properties, in FF 3.6. Excerpts

<div id="wall">
     <div id="small" class="zimg">
       <img src="..." 300x200/>
     </div>
     <br style="clear:both />
</div>

Some CSS

.zimg {
   border:20px solid #ddd;
    background:#eee;
   overflow:hidden;
    position:relative;
}

#wall {
  margin:50px 0 0 50px;
}


#small {
  width:300px;
  height:200px;
  float:left;
}

And a JS function to calculate the relative (x,y) coordinates of the mouse over the IMG

function objabs(o) 
{
   var x = 0,y = 0;

    console.log("get_abs: of "+o.id);
   while (o) {
        console.log("  "+o.id+" x("+o.offsetLeft+") y("+o.offsetTop+")   P:"+(o.offsetParent ? o.offsetParent.id : o.offsetParent));
      x += o.offsetLeft;
      y += o.offsetTop;
      o = o.offsetParent;
   }
   return [x,y];
}

Result: mouse (x,y) are always off by the size of the border (20px).
(console.log() is from Firebug to get a proof!:
smpic x(0) y(0) P:small
small x(50) y(50) P:
x(0) y(0) P:null

50px is the '#wall' margin, border is nowhere.

The border is not taken into account in the calculation - is it a bug (FF or mine)?
Thanks

nb: sorry for the double post but I cannot delete the thread in css - and the JS is probably more appropriate.

Previous Thread

Answer: it is a bug in Firefox, and the function above works as expected in other browsers (Opera...).

So, thanks to the answers on the Css thread, and don't forget to revise your box model ;-)