  - 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 ?  
annotationMode: {
    
    value: 2,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  cursorToolOnLoad: {
    
    value: 0,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  defaultUrl: {
    
    value: "compressed.tracemonkey-pldi-09.pdf",
    kind: OptionKind.VIEWER,
  },
  defaultZoomValue: {
    
    value: "",
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  disableHistory: {
    
    value: false,
    kind: OptionKind.VIEWER,
  },
  disablePageLabels: {
    
    value: false,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },