Howto disable Firefox Drag + Drop



I am trying to implement some custom drag and drop using javascript.
However, in Firefox, the drag and drop extension seems to interfere with
my custom handling of onMouseDown, onMouseMove and onMouseUp.

Once I start to drag the mouse (left-mouse-down + move a few pixels)
firefox shows a drag-forbidden symbol (circle with a diagonal bar
Does anybody know how to disable this firefox extensions as it makes no
sense in my case.

Best Regards