Potentially two stages:

1) Remove US dictionary
a) Look at Tools -> Add-ons

b) Is there a "Dictionary English" listed?

c) If so click on the "Uninstall" button on the Add-on

d) I think you will need to restart Firefox for it to take effect

2) Add a UK dictionary
a) Look at Tools -> Add-ons

b) Click on the "Get Extensions" link at the bottom right.

c) Click on link "dictionaries" under "Browse" on the left

d) Click on the "English (British)" dictionary. It will download.

e) You will need to restart Firefox to install it