Read file from app package without extracting to the filesystem

gistfile1.js

Cu.import("resource://gre/modules/FileUtils.jsm");

XPCOMUtils.defineLazyModuleGetter(this, "NetUtil",
                                  "resource://gre/modules/NetUtil.jsm");   

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 devCertStream = zipReader.getInputStream("dev_cert.pem");
let devCert = NetUtil.readInputStreamToString(devCertStream, devCertStream.available());
devCertStream.close();
debug("Dev cert: " + devCert);