This has nothing to do with search engines, it substitutes the argument from the search bar into the destination url. "wp XYZ" -> "https://en.wikipedia.org/wiki/XYZ"
It has everything to do the browser feature known as "custom search engines", which is a URL replacement capability that was never limited to actual search engines.
Firefox is actually going to remove this feature from bookmarks, and you'll have to create new engines from this page: about:preferences#search