var contentAreaContextMenu = document.getElementById("contentAreaContextMenu");
menuItem = document.createElement("名前");
menuItem.setAttribute("label","名前");
contentAreaContextMenu.insertBefore(menuItem,contentAreaContextMenu.firstChild);
contextMenu.addEventListener("popupshowing", function(event) {
if (event.target != this) return;
呼び出す関数名;
}, false);