I’ve been fighting with Firefox which insists that I’m in Great Britain every time I open it. It continually would tell me I should spell it colour instead of color, and so on.
I finally worded my search correctly on Google and found this tidbit and was able to remove the three dictionaries Firefox had included that were not needed.
In Linux you will find dictionaries in /usr/lib/firefox/dictionaries. You will need root privilegies to delete them.
sudo rm en_GB*
sudo rm en_ZA*
sudo rm en_AU*
This left only the US version.
Restart Firefox and they are gone! Yea!