Deleting an IMAP Folder in Thunderbird
- Tools> Account Settings
- In left pane, select Server Settings
- Select "Remove it immediately" from the drop-down menu next to "When I delete a message." Click OK.
- Delete your folder by right-clicking on it and choosing Delete.
- When you're finished, immediately go back in and change the delete option back to "Move it to the Trash folder" (important!). If you don't change it back to this setting, all of your messages will be deleted permanently when you delete them, instead of just being moved to the Trash.