Firefox British English Dictionary

[#1]

After a recent auto upgrade to Firefox 22, my auto spell check kept on insisting I use the American Version spelling of words.
Most annoying.
I confirmed British English Dictionary on my iMac, in Firefox etc., all to no avail.
I then discovered you can download a British English Version of Firefox, at
http://www.mozilla.org/en-US/firefox/all/
Problem solved !

[#2]

Can you have a look at the Firefox setting - about:config
What does yours say for this key
intl.accept_languages;en-US, en

[#3]

Since I downloaded the British English version, this is what about:config shows...
intl.accept_languages;en-gb
Search term.. intl.accept_languages
after deleting part of your search request, which yielded no result

I also added this Firefox Add-on... British English Dictionary

My Firefox now uses a British English Dictionary.

[#4]

I added the "British Language Pack" under Languages.
For some reason this includes proper English and "South African English".
I disabled the latter.