For Chrisgel:
You probably made the mistake of downloading Firefox from the front page of the Firefox site, instead of clicking on 'Systems and Languages' which would have allowed you to download the British version.
All you have to do to fix your problem is to download this add-on:
https://addons.mozilla.org/en-US/firefox/addon/british-english-dictionary/
Once it's installed, try it out by typing 'colour' into an answer box here on AB. If it's not underlined, the British dictionary is active. (Try typing 'color' to check). If 'colour' is underlined, simply right-click on it and you'll find an option to switch to the British dictionary (which will then work thereafter).
Chris