Thunderbird is great, but very complex and possibly insecure and not private.
Threat model is an important key word here. Imagine you would write Mails over Tor/Tails only and need a secure Mail client.
(Btw I can recommend Carburetor Flatpak for that).
Because of this, the thunderbird hardening user.js, similar to the Arkenfox project exists.
But it is a bit too strict for most threat models. Also settings might change or break, and this has no automatic updating mechanism.
(I should upstream the updater)
The user.js is also just a template, so a ton of mostly not needed configs will stay there.
This project makes the setup of the hardening user.js easy.
Once setup, the script is placed in ~/.local/bin
and a user systemd service runs it every once in a while.
You can comment out lines if you want to keep certain settings.