Here's one I prepared earlier
Ensure Firefox is closed.
Locate the file google.xml among the Firefox program files, probably in \Program Files\Mozilla Firefox\searchplugins\
Using a text editor such as NotePad (not a word processor) change every mention of google.com to google.co.uk, if that is your preferred search injun.
Subsequent searches in Firefox will be returned by the google site of your choice.
It may be wise to save a copy of the updated file in a non-volatile location, as any update to FF will undo your amendments.
(e&oe)