I have been holding off on creating a security/privacy guide for the Thunderbird email client because i've been waiting (and waiting) to see what would happen with a user.js for Thunderbird project on GitHub.
The proposal for a privacy-centric user.js for the bird was first brought up as an issue at the ghacks-user.js for Firefox repository. I was quite interested in such a project, however the months rolled by and not a lot of progress was made and then the original project owner, 'dngray', stopped using Thunderbird.
Today the very kind 'dngray' transferred ownership of the repository to 'HorlogeSkynet' who has expressed an interest in maintaining the project, which i think is great news.
I started writing a Thunderbird Privacy Guide for Dummies! back in May of 2019, but held off on publishing until the user.js project solidified. Now that the day has (hopefully) been calendared, i will be working on finishing my guide.
Stay tuned! And don't forget to subscribe to the 'Thunderbird' category if you want to be informed of updates.