Rated 5 out of 5 stars
Works great on 52.9.1(stock Ubuntu 18.04 thunderbird as of 20180808) - I recognize the add-on is unsupported but it is currently working and this does enable capture of a specific message to a todo list system...good work
Extra note: if you are coming from a Mac system where Mail.app was registered for URIs of format "messageid://<$ID>" so you have a lot of them in your TODO system, you can successfully open the URIs on Ubuntu by registering a script that does this:
LINK=`echo $1 | sed -e 's/message\:\/\/\%3c/thunderlink\:\/\/messageid=/' | sed -e 's/\%3e//'`
/usr/lib/thunderbird/thunderbird -thunderlink "$LINK"