Of course, that will only work for google search. Google maps, google images etc will still likely return localized results.
maqroll: Well, you might also need to restart Fx to see that updated, or at the very least, spawn a new window.
(that is, after you'd edited about:config)
Curious, google.us no longer holds firm. It used to.
For images etc, you could use co.nz or co.uk.