is your account IMAP?

No, it's a POP account. All emails are downloaded to the PC.

Checking where the emails are saved, it appears the program update may have opened a new folder for all new emails and switched the "user interface display" to only show the emails in that directory. How can I add a folder to be displayed on the user interface to be able to check if the "previous" folder still holds my "old" emails?

For some unknown reason it would appear that Thunderbird created a second pop folder for the account and gave you a load of new folders.

You could either try pointing the new accoun to use the old account folder: Right click on pop mail account in Folder Pane and select 'Settings' Select 'Server Settings' under 'Local directory' You may see something similar to this example: C:\Users\User Account\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default\Mail\mail.btinternet-1.com

Notice how it has been given a -1 in the name. This occurs if you have more than one account accessing same server, which could be correct, but if you only have one account accessing that server then it is likely a secons new account of same email address has been created.

Click on 'Browse' button and point to correct old folder. eg: C:\Users\UserAccount\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default\Mail\mail.btinternet.com

click on 'OK' Restart Thunderbird.