クリップボードから取得

function hello(){

  var clip = Components.classes["@mozilla.org/widget/clipboard;1"].createInstance(Components.interfaces.nsIClipboard);

  var trans = Components.classes["@mozilla.org/widget/transferable;1"].createInstance(Components.interfaces.nsITransferable);

  trans.addDataFlavor("text/unicode");

  clip.getData(trans, clip.kGlobalClipboard);
  var str = {};

  var len = {};
  trans.getTransferData("text/unicode", str, len);

  str = str.value.QueryInterface(Components.interfaces.nsISupportsString);

  pastetext = str.data.substring(0, len.value / 2);

  alert(pastetext);

}