[a11y] /challenge-the-default/ page elements marked as aria-hidden are still focusable

[alexgibson]

Description

There are several elements on the /firefox/challenge-the-default/ page that are marked up using aria-hidden="true" but they are still focus-able DOM elements. A focus-able element with aria-hidden="true" is ignored as part of the reading order for screen readers, but still part of the focus order, making it’s state of visible or hidden unclear.

https://dequeuniversity.com/rules/axe/4.10/aria-hidden-focus

Steps to reproduce

1. Open https://www.mozilla.org/de/firefox/challenge-the-default/
2. Notice there are several element on the page using aria-hidden (such as the browser close/X button in the hero), but these are still possible to tab through to using a keyboard.

Expected result

If these visible elements should be hidden from screen readers, then we should make them non-focus-able. Alternatively, if there are elements we could make more accessible then we could remove aria-hidden.

Actual result

Environment

N/A

[alexgibson]

Duplicate of #15137