ARIAMixin not polyfilled on Firefox

[bennypowers]

Consider this example custom element codepen:

<x-menu>Menu</x-menu>
<script type="module">
import 'element-internals-polyfill';

customElements.define('x-menu', class XMenu extends HTMLElement {
  #internals = this.attachInternals();
  constructor() {
    super();
    this.#internals.role = 'menu';
  }
});
</script>

Chromium 111 correctly reports the menu role:

Firefox 110 does not

FWIW, GNOME Web 44 nightly also does not report the menu role, but it's not clear to me why not, since it implements role on ElementInternals

I believe that this happens because the polyfill just checks for the existence of ElementInternals on globalThis and quits when it finds one, assuming the vendor fully implemented everything.