Rated 5 out of 5 stars

per Yannnd el set. 29, 2019 · enllaç permanent

Works great. To use an existing todo.txt: Thunderbird menu → Add-ons → Todo.txt (don't go to Add-ons overview) and a popup shows to add paths to todo.txt and done.txt files.
Hope it will be compatible with Thunderbird 68+ soon!