Create a custom column?

How can I do this in TB78? I didn’t see anything in the TB78 docs about columns.

Thanks,

I think at the moment you’d have to write a WebExtension experiment to do that:
https://thunderbird-webextensions.readthedocs.io/en/78/how-to/experiments.html

The best place to ask about add-on development is here:
https://thunderbird.topicbox.com/groups/addons

Someone may be able to point you to examples of experiments that add columns.