The classic mistake of thinking role=button replaces <button>. It doesn't (as mentioned in the linked PR comments). Native HTML buttons are supported by all user agents and assistive technology and provide keyboard and focus requirements by default [1]. The "short answer" they give as to why they did this (a flexbox bug) links to a comment from February saying this was fixed in Firefox 63.
https://developer.mozilla.org/en-US/docs/Web/Accessibility/A...