Patches Mycroft Project pages to allow adding OpenSearch plugin in Firefox 78+, by overriding the built-in function and replacing the page's own search tags with your choice.

Works with Tampermonkey or Violentmonkey; not sure why it doesn't work in Greasemonkey.

There is an extension which offers a similar fix: https://addons.mozilla.org/firefox/addon/add-search-engine-from-mycroft/