I confirm that the solution works:
sudo ln -s /usr/share/myspell/dicts/ /usr/lib/thunderbird-3.0.4/dictionaries
then restart thunderbird and all dictionaries are now listed.
I confirm that the solution works:
sudo ln -s /usr/share/ myspell/ dicts/ /usr/lib/ thunderbird- 3.0.4/dictionar ies
then restart thunderbird and all dictionaries are now listed.