Firefox redirecting on ondragend event on youtube.

   box = ev.target;

   ev.preventDefault();

   test.style.backgroundColor = "blue";
   document.write("Hello World");

I'm trying to make an addon to firefox, that is supposed to let you 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". The code I'm using is _____________________ //Formatting and creating the test box you drag the video to var test = document.createElement("test"); document.getElementsByTagName("ytd-app")[0].appendChild(test); //the design of the box test.style.color = "black"; test.style.backgroundColor = "white"; test.style.border = "1px solid lightgrey"; test.style.position = "fixed"; test.style.top = "56px"; test.style.right = "0px"; test.style.width = "125px"; test.style.height = "700px"; test.style.zIndex = "10"; //the events that are supposed to get which video is being dragged var box; test.ondragstart = function DragStart(ev) { box = ev.target; } test.ondragover = function DragOver(ev) { ev.preventDefault(); } test.ondrop = function Drop(ev) { test.style.backgroundColor = "blue"; document.write("Hello World"); } test.ondragover = "allowDrop(event)"; __________________ What happends is that whenever I run this on youtube, and drag a video into the box, mozilla firefox redirects to the video, which is what I want to prevent edit: Code and clearification (don't know how to create a code "block"