//  Downloadbutton.uc.js

  (function() {

  if (location != 'chrome://browser/content/browser.xhtml') return;
        
          CustomizableUI.createWidget({
             id: 'Download-button',
             type: 'custom',
             defaultArea: CustomizableUI.AREA_NAVBAR,
             onBuild: function(aDocument) {         
                var toolbaritem = aDocument.createElementNS('http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul', 'toolbarbutton');
                var props = {
                   id: 'Download-button',
                   class: 'toolbarbutton-1 chromeclass-toolbar-additional',
                   removable: true,
                   label: 'Downloads',
                   accesskey: 'D',
                   tooltiptext: 'Downloads',
                   style: 'list-style-image: url(data:image/png;base64,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)',
                   oncommand: "window.open('chrome://browser/content/downloads/contentAreaDownloadsView.xhtml', 'Downloads', 'chrome,resizable=yes,width=600,height=340,left=440,top=250');"
                };
                for (var p in props)
                    toolbaritem.setAttribute(p, props[p]);
                return toolbaritem;
            }
        });

  Downloads.getList(Downloads.ALL)
    .then(list => list.addView({
      onDownloadAdded: () =>
        Services.wm.getMostRecentBrowserWindow().document.getElementById('Download-button').click()
    }));

})();