- browse to about:config
- search for keyword
- set "keyword.enabled" to true
- set "keyword.URL" to https://encrypted.google.com/search?&q=
- search for "browser.fixup.alternate.enabled", set to false
(source : http://support.mozilla.com/en-US/kb/Location%20bar%20search)
ps. you can toggled from true to false (etc) by simply double clicking the entry.
I use opendns, for some reason 1 word searches = opendns search, two word searches = google.