Remove deprecated ::-moz-focus-inner CSS selector
Summary:
Remove the `::-moz-focus-inner` pseudo-element, supposed to "represent an inner focus ring of the <button> element".
It was only ever supported by Firefox; recent Firefox versions do not recognize this CSS anymore, and that creates avoidable noise in the browser console.
The element has become deprecated (instead of experimental) in June 2025; compare
* https://web.archive.org/web/20250617043618/https://developer.mozilla.org/en-US/docs/Web/CSS/::-moz-focus-inner
* https://web.archive.org/web/20250603122556/https://developer.mozilla.org/en-US/docs/Web/CSS/::-moz-focus-inner
Closes T16254
Test Plan: Unknown; probably look at buttons in older Firefox versions.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16254
Differential Revision: https://we.phorge.it/D26318