  - Open https://mozilla.github.io/pdf.js/web/viewer.html
  - Open a pdf with some XFA (for example the on in https://github.com/mozilla/pdf.js/issues/13679#issue-937616350)

The pdf is not correctly displayed because `enableXfa` is false.

@Snuffleupagus, the pref is set to true in Firefox nightly, is there anything blocking setting it to true in the web viewer ?  
/* Copyright 2018 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
const compatibilityParams = Object.create(null);
if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
  const userAgent =
    (typeof navigator !== "undefined" && navigator.userAgent) || "";
  const platform =
    (typeof navigator !== "undefined" && navigator.platform) || "";
  const maxTouchPoints =
    (typeof navigator !== "undefined" && navigator.maxTouchPoints) || 1;
  const isAndroid = /Android/.test(userAgent);
  const isIOS =
    /\b(iPad|iPhone|iPod)(?=;)/.test(userAgent) ||