All versions of Firefox comes with the inline spellchecker (since it existed as of Firefox 2.0). However only a few locales like en-US and maybe a couple others like fr comes with a
dictionary due to licensing reasons.
If you have the en-GB locale of Firefox from
http://www.mozilla.org/firefox/all then you do not need the en-GB language pack. The en-GB Firefox does not come with a dictionary.
The Language Packs is for the language of the UI or User Interface. For example you can have one install of Firefox but then be able to change the language of the UI to different languages. They are not used with the inline spell checker.
https://support.mozilla.org/en-US/kb/ho ... ll-checker
https://support.mozilla.org/en-US/kb/us ... guage-pack