Rated 5 out of 5 stars

Love the potential. Easy to install and use.

The basic version of ChatGPT gives some pretty nonsensical answers that mostly copy something previously written in the email thread, though.

If, in my ChatGPT account, I train my own agent, can I use this instead, or will the vanilla version always be used in Thunderbird?

This review is for a previous version of the add-on (1.1.4). 

If your trained agent can be used with the ChatGPT web interface, you should be able to use it with this addon as well.
Otherwise, you need to wait for the API integration as described here: https://github.com/micz/ThunderAI/issues/40

Thank you.