Was alarmed to learn that Mycroft Project search bar options no longer works, while recreating my Firefox profile. Apparently Mozilla is depreciating Opensearch XML addon support? That's dumb.
https://web.archive.org/web/20200629185533/https://discourse.mozilla.org/t/deprecation-of-opensearch-xml-add-ons-current-users/46868
Found this extension and it does the job of creating custom search bar options perfectly. Highly recommended!