Firefox 3.6.* uses Google's "Browse by name" search rather that "I'm feeling lucky" To set Firefox 4 to use the browse by name search you need to change a hidden preference.
Type about:config into the location bar and press enter
Accept the warning message that appears, you will be taken to a list of preferences
Locate the preference keyword.URL, double-click on it and change it to the link shown below
http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=If you want it to use the I'm feeling lucky search, set it to this link:
http://www.google.com/search?btnI=I%27m+Feeling+Lucky&ie=UTF-8&oe=UTF-8&q=fag.