(function() {

  if (!window.gBrowser)
    return;

  for (let key of document.querySelectorAll('key[keycode="VK_F12"]')) {
    key.setAttribute('disabled', 'true');
  }

  let key = document.createXULElement('key');
  key.id = 'closeTab';
  key.setAttribute('keycode', 'VK_F12');
  key.setAttribute('oncommand', 'gBrowser.removeTab(gBrowser.selectedTab, {animate: true})');
  document.getElementById('mainKeyset').appendChild(key);

})();