Bind a keyword to any search URL so typing <keyword> <query> in Firefox’s address bar searches that site directly.
Settings → Search → Search Shortcuts → Add, then set the URL with %s as the query placeholder. Firefox substitutes %s with whatever you type after the keyword.
Cheatsheet
Raindrop Search Shortcuts
rd -> https://app.raindrop.io/my/0/%s # search Raindrop.io bookmarks