I know it's not the same, but if you bookmark "https://www.youtube.com/results?search_query=%s" and put in a keyword like "yt". You can type "yt whatever" in the addressbar and it will do a search on youtube.
You can also use https://www.reddit.com/r/%S with a capital `%S` to avoid escaping the slashes, so you can type "r all/top" or with GitHub "g Zren/reponame"
This is exactly what I needed. I set up this exact shortcut years ago, in Chrome, and have been slightly annoyed for months that "r sub/top" didn't work on Firefox. Thank you stranger.
And if you set DuckDuckGo or Qwant or a different search engine that supports !bangs as your default search engine, you can use those in your address bar - so no need to configure !yt, !w, !s etc manually.
https://www.givero.com has the full range of DDG bangs - and shares its revenue with good causes.