hahaha.js

var writeIntoFile = function(filepath, content, charset) {
  var file = Components.classes["@mozilla.org/file/local;1"]
                       .createInstance(Components.interfaces.nsILocalFile);
  file.initWithPath(filepath);
  var fostream = Components.classes["@mozilla.org/network/file-output-stream;1"]
                           .createInstance(Components.interfaces.nsIFileOutputStream);

  fostream.init(file, 0x02 | 0x08 | 0x20, 0666, 0);
   
  /* Get a charset converter-enabled stream */
  var costream = Components.classes["@mozilla.org/intl/converter-output-stream;1"]
                           .createInstance(Ci.nsIConverterOutputStream);

  costream.init(fostream, charset, 0, 0x0000);
  costream.writeString(content);

  /* Close everything */
  cosstream.close();
};