By default, Firefox will try to symlink all your hunspell dictionaries in
If you want to have less dictionaries offered to you in Firefox, you can remove some of those links. Be aware that it may not stand an upgrade of Firefox.
In my example, there was whole bunch of English variations displayed, like: English (Australian), English (British), English (Canadian), English (New Zealand), English (South African), etc…
So, the solution as mentioned above is:
# rm /usr/lib/firefox/dictionaries/*
And then go to https://addons.mozilla.org/en-US/firefox/language-tools/ and install only what you need, in my case that was English (US) and Slovenian.