See the bug report for my comment.
http://bugzilla.gnome.org/show_bug.cgi?id=561973#c6 In short, it is a
known fact that GTK+ on Windows doesn't implement generic
inter-process drag-and-drop. Only accepting files dragged from
Explorer onto GTK+ applications work, and dragging images from IE
DnD issues from firefox AFAIK have nothing to do with GIMP... Its the way FF
handles dragging of images. The path resulting from dragging an image is not
a valid file path, because firefox cache is a bit different than IE-s simple
filesytem. In Linux it seems firefox passes URL-s and GIMP fetches
