[tryfuhl]

In the address bar type in "about:config" (without the quotes)

search for keyword.url in the filter field

Replace what's there with:
for a google search use: http://www.google.com/search?q=

or this one to automatically go to first result, like you had it
http://www.google.com/search?btnI=I%27m+Feeling+Lucky&q=