I still miss the ability to add a custom search engine in Firefox Desktop, which has been there in Firefox for Android for years. Kinda weird that Android has an option the desktop version lacks of. Or do I need to enable dev mode or something in the config?
Clicking "find more search engines" only leads to the addons page. On android however, you can add your own site and you can then directly search it from the address bar, for example I added https://www.m.dict.cc/?s=%s to quickly lookup dict.cc for translations, it directly searches for the word I type in.
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You can actually add your website as a search engine. Go to "https://www.dict.cc/?s=", then right-click on a search bar and you will be given the option to add it as a search engine. However, this does not always work. I can't add "https://learn.microsoft.com/ru-ru/search/". So Mozilla should definitely add it.
(Note: a similar idea has been merged into this thread)