in account settings pane, click 'server settings' and UNtick the box to leave messages on server.

I'm running version 115.9.0. I want to completely remove an account, including all the data on the TB server. In Account Settings, I don't see the option to untick the "leave messages on server" under the account I want to completely delete. Is this setting in a different place on this version of Thunderbird?

The best place to eliminate an email account is at the email provider's website. That deletes everything. To do the nearest equivalent with thunderbird, you would: - highlight all messages on the account and delete them. Then empty the trash folder. - next, click to account settings, highlight and remove the account. There will be a final prompt on removing any account data. Click remove and that will cause any folders on the PC to be removed. - BUT the account itself still exists on the email provider's site. And would still collect any incoming messages.