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 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  mkdir  /usr/lib/thunderbird/dictionaries/

sudo mv  /usr/lib/thunderbird/dictionaries.back/en_GB.* /usr/lib/thunderbird/dictionaries/

And restart Thunderbird.