gecko/browser/base/content/test/head.js

function updateTabContextMenu(tab) {
  let menu = document.getElementById("tabContextMenu");
  if (!tab)
    tab = gBrowser.selectedTab;
  menu.openPopup(tab, "end_after", 0, 0, true, false, {target: tab});
  is(TabContextMenu.contextTab, tab, "TabContextMenu context is the expected tab");
  menu.hidePopup();
}