var newHdrVal = curHdr.childNodes[1].inputField.value;
if (curHdr.converted) {
    var mimeEncoder = Components.classes["@mozilla.org/messenger/mimeconverter;1"].getService(Components.interfaces.nsIMimeConverter);
    var unicodeConverter = Components.classes["@mozilla.org/intl/scriptableunicodeconverter"]
          .createInstance(Components.interfaces.nsIScriptableUnicodeConverter);
    var aCharset = curHdr.cs; 
    unicodeConverter.charset = aCharset;
    newHdrVal = mimeEncoder.encodeMimePartIIStr(unicodeConverter.ConvertFromUnicode(newHdrVal), curHdr.structured, aCharset, 0, 72);
}