Read file from app package with extracting to the filesystem first

gistfile1.js

let app = DOMApplicationRegistry.getAppByManifestURL(manifestURL);
if (!app) {
  // handle
}

let appDir = FileUtils.getDir("coreAppsDir", ["webapps", app.id], false);
let appPackage = appDir.clone();
appPackage.append("application.zip");

let zipReader = Cc["@mozilla.org/libjar/zip-reader;1"]
                  .createInstance(Ci.nsIZipReader);

zipReader.open(appPackage);
debug('has file: ' + zipReader.hasEntry("dev_cert.pem"));

let extractedDevCert = appDir.clone();
extractedDevCert.append("dev_cert.pem");
zipReader.extract("dev_cert.pem", extractedDevCert);

let devCertChannel = NetUtil.newChannel(extractedDevCert);
devCertChannel.contentType = "text/plain";

NetUtil.asyncFetch(devCertChannel, function(aStream, aResult) {
  if (!Components.isSuccessCode(aResult)) {
    debug("Could not read file " + extractedDevCert.path);
    return;
  }

  debug("File read!!");

  let devCert = NetUtil.readInputStreamToString(aStream, aStream.available());

  debug("Dev cert: " + devCert);
});