If you have created a POP mail account in Thunderbird, then POP mail accounts can only access the server Inbox to download mail.

If you have emails in the server 'Sent' folder - as seen when you logon to your webmail account - then it is possible to get them downloaded, but you would need to logon to webmail account via a browser and move those emails from the 'Sent' folder into the 'Inbox' folder. Then in Thunderbird click on 'Get Messages' to download them. Finally, move those 'sent' messages from Pop account Inbox into Pop account 'Sent' folder. And also in webmail account move those 'sent' messages back into 'Sent' folder - assuming in Thunderbird you have previously set the option to 'Leave messages on server'.