Issue with document.requestStorageAccess on Firefox

I just followed this example https://developer.mozilla.org/en-US/docs/Web/API/Document/requestStorageAccess#examples to request localStorage access for a cross domain iframe. This works in Chrome. but in Firefox the handle is undefined. I can confirm that the localStorage access has been granted. Any idea why the handle is undefined in Firefox

document.requestStorageAccess({ localStorage: true }).then(
  (handle) => {
    console.log("localStorage access granted");
    handle.localStorage.setItem("foo", "bar");
  },
  () => {
    console.log("localStorage access denied");
  },
);