Thanks for your post, Nungate.
If, when you downloaded Firefox, you went via the link that says 'Systems and Languages' you'll have found and downloaded the
British version. However most people simply click on the 'Download' button on the Firefox home page, getting them the American version.
Try typing 'colour' and 'color' into an AB answer box. If 'color' is underlined as incorrect you've got the British spellchecker. If 'colour' is marked as incorrect you've got the American spellchecker. If so, see my post at 1410 here to correct the problem:
http://www.theanswerbank.co.uk/ChatterBank/Question1224525.html#answer-7819597
Chris