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Drag & Drop

Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

Dropping files onto an XUL application

It's possible to setup drag and drop events to handle dropping files from external applications or OS file managers onto your XUL-based application. First, you need to hook up the basic drag event handlers:

elem.addEventListener("dragover", _dragOver, true);
elem.addEventListener("dragdrop", _dragDrop, true);

Here, elem could be a window or an XUL element. Next, setup the handlers so that files can be dropped on the application:

  function _dragOver(aEvent) {
    var dragService = Components.classes[";1"].getService(Components.interfaces.nsIDragService);
    var dragSession = dragService.getCurrentSession();

    var supported = dragSession.isDataFlavorSupported("text/x-moz-url");
    if (!supported)
      supported = dragSession.isDataFlavorSupported("application/x-moz-file");

    if (supported)
      dragSession.canDrop = true;

  function _dragDrop(aEvent) {
    var dragService = Components.classes[";1"].getService(Components.interfaces.nsIDragService);
    var dragSession = dragService.getCurrentSession();
    var _ios = Components.classes[';1'].getService(Components.interfaces.nsIIOService);
    var uris = new Array();

    // If sourceNode is not null, then the drop was from inside the application
    if (dragSession.sourceNode)

    // Setup a transfer item to retrieve the file data
    var trans = Components.classes[";1"].createInstance(Components.interfaces.nsITransferable);

    for (var i=0; i<dragSession.numDropItems; i++) {
      var uri = null;

      dragSession.getData(trans, i);
      var flavor = {}, data = {}, length = {};
      trans.getAnyTransferData(flavor, data, length);
      if (data) {
        try {
          var str = data.value.QueryInterface(Components.interfaces.nsISupportsString);
        catch(ex) {

        if (str) {
          uri = _ios.newURI("\n")[0], null, null);
        else {
          var file = data.value.QueryInterface(Components.interfaces.nsIFile);
          if (file)
            uri = _ios.newFileURI(file);

      if (uri)

    // Use the array of file URIs

The _dragOver function checks the drag data to see if a simple text file or general purpose file types are available. If the file types are present in the drag data, the function returns that dropping the data is allowed. The _dragDrop function retrieves the file URI data from the drag session and creates an array of file URIs that the XUL application can use.