To repeat a previous user's advice (thanks), the issue seems to be that the link Mozilla provides links to the below URL but with "en-US" where "en-GB" is. If you change the address, it works fine.

addons.mozilla.org/en-GB/firefox/addon/english-gb-language-pack/

How Mozilla hasn't picked up on this for so long is beyond me. If the technical cause of the issue is what I suspect it is, it'd be a straightforward fix.

"strict_max_version": "142.*"

Why the hell do you set a MAX version, when Nightly uses v143? I can't use a fucking language pack because I use a newer version?

Werkt niet.

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.

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.