gecko-dev/content/base/test/send_gzip_content.sjs

const Cc = Components.classes;
const Ci = Components.interfaces;

function gzipCompressString(string, obs) {

  let scs = Cc["@mozilla.org/streamConverters;1"]
           .getService(Ci.nsIStreamConverterService);
  let listener = Cc["@mozilla.org/network/stream-loader;1"]
                .createInstance(Ci.nsIStreamLoader);
  listener.init(obs);
  let converter = scs.asyncConvertData("uncompressed", "gzip",
                                        listener, null);
  let stringStream = Cc["@mozilla.org/io/string-input-stream;1"]
                    .createInstance(Ci.nsIStringInputStream);
  stringStream.data = string;
  converter.onStartRequest(null, null);
  converter.onDataAvailable(null, null, stringStream, 0, string.length);
  converter.onStopRequest(null, null, null);
}

function produceData() {
  var chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()_+";
  var result = '';
  for (var i = 0; i < 100000; ++i) {
    result += chars;
  }
  return result;
}

function handleRequest(request, response)
{
  response.processAsync();

  // Generate data
  var strings_to_send = produceData();
  response.setHeader("Content-Type", "text/plain", false);
  response.setHeader("Content-Encoding", "gzip", false);

  let observer = {
    onStreamComplete: function(loader, context, status, length, result) {
      buffer = String.fromCharCode.apply(this, result);
      response.setHeader("Content-Length", ""+buffer.length, false);
      response.write(buffer);
      response.finish();
    }
  };
  gzipCompressString(strings_to_send, observer);
}