What have you tried? Right click is the usual way in.

I do the right click and the folders appear to be gone; but as soon as I close Thunderbird and re start it the folders are back!

Is this a Pop or IMap mail account?

What folder are you trying to delete? Is it a standard default Thunderbird folder or one you created?

If IMAP account folder that you created, have you tried to unsubscribe from seeing the folder and then deleting it via webmail account.

I created the folders in Thunderbird. I don't know how to tell whether folder is POP or IMap mail.

How to tell if pop or Imap: Tools > Account Settings > Server Settings for the mail account or menu icon > Options > Account Settings > Server Settings for the mail account or right click on the mail account name in the left Folder Pane and select 'Settings' then select the 'Server Settings' for the mail account.

At the top it will say: Server Type: Pop mail server or Imap mail server