Drag And Drop zone not displayed in IE and Firefox

On IE9, the drop zone, usually a gray background with a dashed border, is not displayed at all when you drag a document.
With Firefox 12.0, the zone (background+border) is displayed only when you roll over the border (1 or 2 pixels), and the dropzoneHL class disappear once you're in the zone.

The .dropzoneHL class, in charge of display the gray zone + the dashed border, is only added when you rollover the border, not the content. That is why you see the zone only one second in Firefox.