If anyone has tried to install the English GB Dictionary on Firefox 18.104.22.168 you'll probably find it's not working, it says it's not compatible. The script for that file hasn't been updated, I have rewritten the script for it and uploaded it to my web site. Let me know and I'll post the link to it.
It sounds like you are lucky, there's a few other people who are having problems with it, all the other languages have been updated, but not the en-GB one for some reason, no doubt they'll realise and change it eventually. You can disable the extension compatibility check in about:config but it disables compatibility checking for all the extensions which can cause a problem.
That's interesting because when I upgraded to 22.214.171.124 it checked the compatibility and it threw my GB spellcheck dictionary as not compatible, that's why I had to dabble around with the one off the Mozilla site.
I had the same problem when I installed Firefox 126.96.36.199 on my hard drive after a reformat. To solve it I used the manual method described by Pedestrian on this Mozilla forum click here. My C:\Program Files\Mozilla Firefox\ folder didn't have a dictionaries folder so I created one and copied the en-GB.aff and en-GB.dic files to it. After closing and reopening Firefox the dictionary was there. Hopefully as a result there aren't any spelling errors in this posting!
I saw that thread as well and did the same as you and copied both those files across, restarted and it still wouldn't work, I do use Linux that might be something to do with it, but can't imagine what. I even tried the other method by unpacking the file and editing the install.rdf file then resaving the file as a zip then renaming the zip to xpi, nope still wasn't having it, it told me it couldn't find the install file. What I had to do in the end was to edit the install.rdf file within the archieve application and keep it as an xpi file, if that makes sense.