var i; var m; var targetTab;
var links = [];

this.ondragover = () => false;
this.ondrop = e => {
    i=-1;
    var place = e.dataTransfer.getData("text/x-moz-place"), obj;
    if (place && (obj = JSON.parse(place)).type == "text/x-moz-place-container"
        && !("annos" in obj && obj.annos.some(anno => anno.name.startsWith("livemark/") || anno.value == "AllBookmarks"))) {
        var html = e.dataTransfer.getData("text/html");
        var doc = new DOMParser().parseFromString(html, "text/html");
        links = [...doc.querySelectorAll("body > DL > DD > A")].map(a => a.getAttribute("HREF"));
       }
       
  m = links.length;  
  stop(); 
}

this.leftclick = function(event) {stop();}

function stop(){
        if(i != -1){
            targetTab = gBrowser.selectedTab; gBrowser.removeTab(targetTab);
            targetTab = gBrowser.selectedTab; gBrowser.removeTab(targetTab);
            targetTab = gBrowser.selectedTab; gBrowser.removeTab(targetTab);
            }
    i++; if(i <= m-1) gBrowser.selectedTab = gBrowser.addTab(links[i]); else return;
    i++; if(i <= m-1) gBrowser.addTab(links[i]); else return;
    i++; if(i <= m-1) gBrowser.addTab(links[i]); else return;
   }

this.setAttribute('onclick', 'custombuttons.gQuot.mHandler(event, this)');
this.tooltipText = "drag'n'drop - folder:\n"+"<--- Начать просмотр";

addEventListener('mousedown', e => {
  if (e.button != 0) return;

  let tab = e.originalTarget;
  while (tab && tab.localName != 'tab')
    tab = tab.parentNode;
  let controlItem = e.originalTarget.localName == 'toolbarbutton' || e.originalTarget.localName == 'image';

  if (!tab || controlItem) return;

  let targetTab = tab._tPos - 1;

  gBrowser.visibleTabs.forEach(tab =>
                               tab._tPos == targetTab
                               && tab.hasAttribute("busy")
                               && tab.linkedBrowser._webNavigation.stop(1))
}, true, gBrowser.tabContainer);

function isBusy() {
    if (gBrowser.visibleTabs.filter(tab => tab.hasAttribute("busy")).length == 0)
        gBrowser.addTab(links[i]);
}
addEventListener("TabAttrModified", isBusy, false, gBrowser.tabContainer);

removeEventListener("TabAttrModified", isBusy, false, gBrowser.tabContainer);

this.tooltipText = "Перетащите папку закладок на кнопку:\n"+
                   "ссылки из папки будут загружаться по очереди.\n"+
                   "Остановить загрузку - ЛКМ.";

var i; var m; var links = [];

this.ondragover = () => false;
this.ondrop = e => {
    var place = e.dataTransfer.getData("text/x-moz-place"), obj;
    if (place && (obj = JSON.parse(place)).type == "text/x-moz-place-container"
        && !("annos" in obj && obj.annos.some(anno => anno.name.startsWith("livemark/") || anno.value == "AllBookmarks"))) {
        var html = e.dataTransfer.getData("text/html");
        var doc = new DOMParser().parseFromString(html, "text/html");
        links = [...doc.querySelectorAll("body > DL > DD > A")].map(a => a.getAttribute("HREF"));
       }
  
  i=0;    m = links.length; 
  
  run();
}

function run(){
    if (gBrowser.visibleTabs.filter(tab => tab.hasAttribute("busy")).length == 0)
          if (i <= m-1){gBrowser.addTab(links[i]); i++;}
        else return; 
    
    setTimeout(()=>run(), 500);
}

this.onclick = function(e){
  if (e.button == 0) i = 10000; 
}

gBrowser.visibleTabs.filter(tab => tab.hasAttribute("busy")).length
gBrowser.visibleTabs.some(tab=>{ return tab.getAttribute("busy") })

[...gBrowser.tabs].some(tab=>{ return tab.getAttribute("busy") })