When using IMAP protocol you have to Subscribe to folders on the server that you want to see in Thunderbird and Unsubscribe to the ones you don't want to see.

Sent is more than likely a local folder created by Thunderbird and Sent Items is one created on your providers server. Go to the File menu and click Subscribe. Subscribe and Unsubscribe as needed. Then go to Account Settings-Copies and Folders and tell Thunderbird to save sent messages in the Sent Items folder on the server instead of the Sent folder.

Thank you! Tb now displays the "Sent Items" folder.

I must note that

1) Tb will delete messages in its displayed "Sent Items" folder, but If I log onto my account through its webpage interface, the folder named "Sent Items" still contains messages that I deleted with Tb. I can delete the "Sent Items" messages through the webpage interface.

2) When Tb sends a message, there are two (2) copies of it in its displayed "Sent Items" folder. (See attached image.) The files are different: the first contains header fields generated by the mail server, the second contains the correct destination header field. The webpage interface shows two (2) copies that appear identical except for different send times.