For everyone looking for the actual language pack that shows up correctly in your options: its here: addons.mozilla.org/en-GB/firefox/addon/british-english-dictionary-gb/
i dont know what this is meant to do, but it sure doesn't show up as a language to pick. even when restarting firefox. however the above link worked first try.

Downloaded and switched to English (GB) in my Firefox settings but it still uses English (US) :/

Does not work: always defaults to US English. Very annoying.

Love these language packs, I have different versions for English, as well as my mother tongue french.I also have my other languages that I am slowly learning. Our team we have another service that we love but honestly this tool is a must when working with Mozilla Firefox : it never fails me.

exclent easy 4star