Yes, this feature is removed in 78.

Thanks a lot for your answer, TyDraniu.

Do you know if there is any alternative to Mycroft?

The ability to use different search engines easily from the search bar is one of the reasons Firefox is so useful for me.

Hi Marcos, the window.external.AddSearchProvider() function no longer works in Firefox 78+. I emailed Mycroft Project with a suggested patch to their site to integrate with Firefox's built-in Autodiscovery feature (<link rel="search"> tag) so hopefully they will do that. Meanwhile, you can use an add-on to adapt Mycroft to Autodiscovery. Either:

• Stand-alone extension: https://addons.mozilla.org/firefox/addon/add-search-engine-from-mycroft/
• Userscript for Tampermonkey or Violentmonkey: https://greasyfork.org/en/scripts/405759-mycroft-patch-for-firefox-78 (I posted this)

Thank you so much, jscher2000.

I've just tried the first method, and it works just fine :)

Apparently the first option, the Addon https://addons.mozilla.org/firefox/addon/add-search-engine-from-mycroft/ it's not working anymore. The page appears but there is no link functionality to click. I am using Firefox 82.0.2 (32-bit). I've been using Firefox since 2002. I myself created about 15 engines on Mycroft. This limitation created by Mozilla is somewhat irritating. Has any change occurred?

Scratch that.

The site has been updated recently to implement the same strategy as the add-ons. After you click a link, Mycroft displays a new page explaining how to install the search engine plugin from the search bar drop-down, and you also can use the Page Actions menu in the address bar.

Example screenshots attached.

I even didn't notice that we don't need this addon anymore :).