url_wrangle.js

function urlWrangle(aUrlStr) {
  const ioService = Cc["@mozilla.org/network/io-service;1"]
                    .getService(Ci.nsIIOService);
  let uri = ioService.newURI(aUrlStr);

  if (uri.schemeIs("resource")) {
    const ph = ioService.getProtocolHandler("resource")
               .QueryInterface(Ci.nsISubstitutingProtocolHandler);
    uri = ioService.newURI(ph.resolveURI(uri));
  } else if (uri.schemeIs("chrome")) {
    const reg = Cc["@mozilla.org/chrome/chrome-registry;1"]
                .getService(Ci.nsIChromeRegistry);
    uri = reg.convertChromeURL(uri);
  }

  if (uri.schemeIs("file")) {
    let nsif = uri.QueryInterface(Ci.nsIFileURL).file;
    return File.createFromNsIFile(nsif)
               .then(URL.createObjectURL);
  }
  if (uri.schemeIs("jar")) {
    return fetch(uri.spec)
           .then(resp => resp.ok ? resp.blob()
                                 : Promise.reject(new Error(resp.statusText)))
           .then(URL.createObjectURL);
  }
  return Promise.resolve(uri.spec);
}

function loadFrameScriptSafe(mm, url, delayed) {
  urlWrangle(url).then(safeUrl => mm.loadFrameScript(safeUrl, delayed));
}