Creating a new img element and cannot change style.left or style.top to set placement as I can in other browsers

Question

I create a new element of type img and am placing in a specific absolute position by setting style.position, style.left and style.top attributes. Works fine in IE and Opera but FF does not allow me to modify any of the style attributes either before or after the element is inserted into the document. I've tested this with 3.5.9 and 3.6.3 and both have the problem.
== This happened ==
Every time Firefox opened
== upon install of FF

Answer 1

I knew it was something incredibly obvious that I was missing.
by doing
himg.style.left = (Math.floor(coords[0]) + tweeks.x) + "px";
himg.style.top = (Math.floor(coords[1]) + tweeks.y) + "px";
instead of
himg.style.left = Math.floor(coords[0]) + tweeks.x;
himg.style.top = Math.floor(coords[1]) + tweeks.y;
it works. It seems Opera and IE assume px if nothing supplied while FF needs it explicitly specified.
Thanks!