With the FastAwesomeBarSearch add-on Firefox address bar can act as a search box. You can type here and press enter to search.
Sometimes some plugin/add-on change this default search from Google to Ask or something else.
Also you want to add "I'm feeling lucky" feature to this search. The benefit is when you will search some site and Google is confident enough that it has found the site you are looking for, it will directly take you to the site.
To fast search in Firefox you can do it manually by going to about:config and adding/changing the key "keyword.url" to this value "http://www.google.com/search?btnI=I'm+Feeling+Lucky&q=".
Or you can install this add-on.
· Mozilla Firefox
What's New in This Release: [ read full changelog ]
· Added restoring search to previous state when disabled/removed.