You can set a keyword for any bookmark. Firefox replaces %s with your query. Simply bookmark the search result page of any website, replace the query with %s and set the keyword for the bookmark.

Doesn't have to be search either. For instance, I use it for the nodejs docs, and npm packages too: "https://nodejs.org/api/%s.html" and "https://www.npmjs.com/package/%s". So I can type "node fs" open the fs docs for node, or "npm fs-extra" to look up the docs for "fs-extra" package on npm.