Adds YouTube search engine to your firefox browser. In URL bar you could just type "@youtube search_query".
I'm developing also another great extension which is a multi search engine, currently in development. Take a look at it here. I recommend it.
If you want follow what I'm currently working and developing... Follow my GitHub account. Stay tuned!..
Firefox has a chrome_settings_overrides.search_provider to add your search
engine to the omnibox. Please, take a look at
MDN docs.