  - 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 ?  
value:
      typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")
        ? "../src/worker_loader.js"
        : "../build/pdf.worker.js",
    kind: OptionKind.WORKER,
  },
};
if (
  typeof PDFJSDev === "undefined" ||
  PDFJSDev.test("!PRODUCTION || GENERIC")
) {
  defaultOptions.disablePreferences = {
    
    value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("TESTING"),
    kind: OptionKind.VIEWER,
  };
  defaultOptions.locale = {
    
    value: typeof navigator !== "undefined" ? navigator.language : "en-US",
    kind: OptionKind.VIEWER,
  };
  defaultOptions.sandboxBundleSrc = {
    
    value:
      typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")
        ? "../build/dev-sandbox/pdf.sandbox.js"
        : "../build/pdf.sandbox.js",
    kind: OptionKind.VIEWER,
  };