Firefox redirecting on ondragend event on youtube.

I'm trying to make an addon to firefox, that is supposed to let your create a custom, temporary playlist on youtube, by dragging and dropping the video you want to add to this temporary playlist, but whenever I try to drag and drop a video on the box that I created (to test if my code works so far), firefox automatically plays this video (unless I have an alert), so I wonder if it's possible to cancels firefox redirect in some other way than an alert, since it's pretty "unsmooth".