SearchAutocomplete, is a WebExtension add-on for Firefox that aims to give you quick, keyword based autocomplete for your search engines. Making your address bar an effective tool again ;-). Should be a good replacement for Omnibar add-on (that worked until FF 56).
The add-on works in the address bar. Just go to the address bar and type in sa
and your search engine keyword (after space). For example when you type in sa en Some
in the address bar you will see a list of articles starting with "Some" from English Wikpedia.