请教各位在Firefox上如何使用document.execCommand()？？？

function copyToClipboard(txt) {
     if(window.clipboardData) {
             window.clipboardData.clearData();
             window.clipboardData.setData("Text", txt);
     } else if(navigator.userAgent.indexOf("Opera") != -1) {
          window.location = txt;
     } else if (window.netscape) {
          try {
               netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
          } catch (e) {
               alert("被瀏覽器拒絕！\n請在瀏覽器網址列輸入'about:config'\n，將'signed.applets.codebase_principal_support'設為'true'");
          }
          var clip = Components.classes['@mozilla.org/widget/clipboard;1'].createInstance(Components.interfaces.nsIClipboard);
          if (!clip)
               return;
          var trans = Components.classes['@mozilla.org/widget/transferable;1'].createInstance(Components.interfaces.nsITransferable);
          if (!trans)
               return;
          trans.addDataFlavor('text/unicode');
          var str = new Object();
          var len = new Object();
          var str = Components.classes["@mozilla.org/supports-string;1"].createInstance(Components.interfaces.nsISupportsString);
          var copytext = txt;
          str.data = copytext;
          trans.setTransferData("text/unicode",str,copytext.length*2);
          var clipid = Components.interfaces.nsIClipboard;
          if (!clip)
               return false;
          clip.setData(trans,null,clipid.kGlobalClipboard);
     }
     alert('已經複製');
}