Thunderbird creates a folder called INBOX on IMAP
Go to the Config Editor and enter trash_folder. Hopefully one of the items listed is mail.server.server#.trash_folder_name. If the value is INBOX/Trash change it to Trash. Exit from Thunderbird, then restart it. If the Trash folder now has a recycle bin icon, then it worked and you should be able to delete the INBOX and INBOX.Trash folders.
I found the relevant details in http://kb.mozillazine.org/IMAP_Trash_folder, but I used the config editor rather than editing prefs.js directly.