I haven't yet tried FF3, but I'm sure there must be a way, considering how customizable FF has been in the past. Why do you want to? Does it default to something other than Google? That would be crazy, if it did!
Thanks clevin. I already tried looking around about:config but didn't find the relevant entry.
The string to insert is "http://eu3.ixquick.com/do/metasearch.pl?language=english&cat=web&query=" in case anyone else is interested in abandoning Google and enjoying their privacy