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.