Sounds like it is Acting as if you selected 'Edit new Message' or 'Ctrl+E'.

Do you usually view the email in the lower 'Message Pane' or in a new tab? Is the 'Message Pane' enabled or not enabled?

Are you using a MAC or Windows OS computer? Is it a latop or do you have a separate keyboard?

What version of Thunderbird are you using?

re :When I open an email message, How are you doing this? Are you using a mouse and using one left click on the email in the list ? Or double click on email in list to open in a new tab? Or Message Pane is disabled, so you select email and then click on 'Enter'/Return' key to display in new tab?

There was an old bug reported on this but it did seem to effect MAC users, but that was a few versions ago. In that instance the user was clicking on the 'Enter/Return' key to open an email.

Please answer all questions in the first comment so we get a better understanding of your setup.

In the meantime in order to read emails, a workaround might be to use the keyboard shortcut for opening messages, Ctrl+O or Command+O (mac), or right click on email in list and select 'Open Message in new tab' from the context menu.

Thanks for the work-around. To answer you questions:

I use a desktop PC with separate keyboard.

Thunderbird version is 78.7.1

I usually double-click on the email message which opens a new tab and do not use the Message Pane. The Message Pane is not enabled.

This started happening on Feb. 10. I downloaded Thunderbird again to see if that would correct the problem but it didn't.

Thanks!

re :I use a desktop PC with separate keyboard.

What Operating System are you using ? MAC OS 10.11.6, Windows10, Linux ? Is keyboard a mac keyboard ? eg: Apple A1243 extended keyboard

I use Windows 10. The keyboard is a Logitech K350.

have a look in the copies and folders part of account settings for the account. Is the inbox set to the drafts folder by some mischance?

Problem solved! Many thanks! I had just added new email accounts and clicked on the wrong button in the Account Settings.