When you first drag the red div element (below), Firefox renders a ghost-image that is exactly the same size as the source div.
However, if you resize the div element (via the resize-grip in its bottom-right corner) to a "large width" or "large height", firefox renders a ghost-image that is much smaller than the source div element. Chromium and Chrome do NOT do this.
This resizing behavior is problematic in applications where the user will be dropping large elements to an absolute position; it causes the user to have to make several attempts before they can align the element exactly to their desired position. Click here for a video demonstration. A bug report has been submited here and a work-around has been request at stackoverflow.