Having 5 unused English dictionary is annoying (I only need one, not en_NZ, en_US, en_AU and en_ZA)
First remove dictionary:
sudo /usr/lib/thunderbird/dictionaries.back/ sudo mv /usr/lib/thunderbird/dictionaries/* /usr/lib/thunderbird/dictionaries.back/
and prevent them from being reinstall the next update:
sudo dpkg-divert --add /usr/lib/thunderbird/dictionaries
then place only the wanted dictionary:
sudo mv /usr/lib/thunderbird/dictionaries.back/en_GB.aff /usr/lib/thunderbird/dictionaries/en.aff sudo mv /usr/lib/thunderbird/dictionaries.back/en_GB.dic /usr/lib/thunderbird/dictionaries/en.dic
And restart Thunderbird.