In general, you can delete a folder by right clicking on it and choosing Delete. It will be treated according to your settings the same as if it was a message, i.e. moved to Bin/Trash, not deleted immediately, if those are your settings for deleting messages.

If this is being treated as a special folder by Thunderbird, you should make it a normal folder first, though. Do both folders have the same icon in Thunderbird?

Is this an IMAP account you access from other applications or devices? Apple Mail calls the folder Archive, whereas Thunderbird calls it Archives by default, although you may configure it to use any other folder you want for that purpose. Something like that could explain why you have both folders.

It is an IMAP account which is accessed by other devices. But I have other IMAP accounts on the same TB application which do not have this problem. How do I turn a special folder into a normal folder? Both folders have the same icon.

Right-click the account in the Folder Pane, Subscribe..., and uncheck (unsubscribe) the Archive folder.

If the unwanted folder has the special Archives icon, it's because it has been flagged as such, normally as a result of having chosen it as the folder to be used for that purpose. If no account has been configured to use that folder for archives, you may remove the flag by choosing it as the folder to be used for archives, then choosing another folder for that purpose.