To better help you, please post here a screenshot showing us what you are seeing. How do I create a screenshot of my problem?

I tried uploading a screenshot with my original post and am trying again right now. All that happens is that the little wheel spins endlessly but no upload occurs. I thought the image might be too big, but it's 400dpi, which shouldn't be a problem. The page that appears when I open a subfolder has two banners. The top one says "Accounts" with links for View Setting for This Account" and Create a New Account. The one in mid-page says "Advanced Features," with links for Search Messages and Manage Message Filters. I appreciate your help.

There are two files in your Thunderbird Profile folder that I want you to delete. One or both of them may be corrupted. They will both be rebuilt when Thunderbird starts again.

Find & open your Thunderbird Profile folder: Thunderbird menu: Help: Troubleshooting Information (If you cannot see the Thunderbird menu, press the Alt key or F10 key on your keyboard to show the menu) Under "Application Basics", beside "Profile Folder", click the "Show Profile" button.

This will open a Windows Explorer window showing the contents of your Thunderbird Profile folder.

Keep this Windows Explorer window open, but quit Thunderbird.

Delete these two files:

panacea.dat
global-messages-db.sqlite

Then start Thunderbird. It might take some time to re-index the folders, so be patient.