var file = Components.classes["@mozilla.org/filelocal;1"].createInstance(Components.interfaces.nsILocalFile);
file.initWithPath("Pathforfile");
file.create(Components.interfaces.nsIFile.NORMAL_FILE_TYPE, 420 );
var outputStream = Components.classes["@mozilla.org/network/file-output-stream;1"].createInstance( Components.interfaces.nsIFileOutputStream );
outputStream.init( file, 0x04 | 0x08 | 0x20, 420, 0 );
var output="Texttowriteinfile";
var result = outputStream.write( output, output.length );
outputStream.close();