After struggling every time I want to install new dictionaries in thunderbird, I think its time to write it down. Once and for all. It is as simple as that: Take a dictionary in the appropriate format (*.{dic,aff}, for exmple from the thunderbird homepage or openoffice myspell packages) and drop it into /usr/lib/mozilla-thunderbird/dictionaries. Done. The xpi packages do not work for me, neither installing into either /opt/thunderbird/components/myspell/, nor /usr/lib/mozilla-thunderbird/components/myspell.