Tamborine's post seems to refer to the way that Windows works, not to the way that Firefox does.
Go to Tools > Add-ons > Extensions and check that 'British English Dictionary' is listed and not shown as 'Disabled'. Your next course of action depends upon what you've found:
1. If it's listed (and not disabled), come to an AB post and type 'colour' into the reply box, followed by a space. It will then be underlined as an error, so right-click on it and then on Languages > English (United Kingdom). That should fix the problem.
2. If it's listed but disabled, click on 'Enable'. Then try typing 'colour', as above, and (if necessary) switching to UK English (also as above).
3. If it's not listed, search for 'British English Dictionary' in the search box at the top right of the 'Extensions' page. Install it and then try the 'colour' test, as above.