Zum letzten aktiven Tab wechseln und Tab duplizieren

(function() {

  if (!window.gBrowser)
    return;

  let previousTab = gBrowser.selectedTab;
  let selectedTab = gBrowser.selectedTab;

  gBrowser.tabContainer.addEventListener('TabSelect', function(event) {
    setTimeout(function() {
      previousTab = event.detail.previousTab;
      selectedTab = event.target;
    }, 400);
  });

  function getTab(node) {
    while (!node.classList.contains('tab-close-button') && !node.classList.contains('tab-icon-sound')
           && node.localName != 'tab' && node != gBrowser.tabContainer) {
      node = node.parentNode;
    };
    return node.localName == 'tab' ? node : null;
  };

  let selectPreviousTab;

  gBrowser.tabContainer.addEventListener('mousedown', function(event) {
    selectPreviousTab = false;
    if (event.button == 0 && getTab(event.target) == selectedTab) {
      if (event.detail == 1) {
        selectPreviousTab = true;
      } else if (event.detail == 2) {
        duplicateTabIn(selectedTab, 'tab');
      }
    }
  });

  gBrowser.tabContainer.addEventListener('click', function(event) {
    if (selectPreviousTab) {
      gBrowser.selectedTab = previousTab;
    };
  });

})();