There is no 'preview pane'. When a message is viewed in the Message Pane, it has been 'fully' opened. If some other app describes the display of the first few lines of a message as a 'preview', the message has in fact been opened.

If you want to move, delete etc. messages without opening them, close the Message Pane (F8).

Thank you. Pressing F8 on my latest version (as of today) Thunderbird causes a window to open on the right side of the screen entitled PROJECTS.

The F8 key simply toggles the Message Pane, and it should always display the selected message. If that isn't what you see, run in safe mode (hold Shift when launching TB) to test for a broken layout (fixed by'Reset toolbars & controls' at the safe mode prompt).

This is what shows up on the right side of my screen when I press F8.

It seems the F8 key is bound to another process, i.e. connecting to a 2nd monitor. Try enabling the Message Pane from the 3-bar menu icon, Customize, Layout.

Could be my new Dell won't allow Thunderbird to change what the F8 key does. When I go into the view you suggest it does say F8 next to Message Pane. Anyway, I see how to disable the pane.

In Windows Live Mail you could set it to mark a message as read (after a selectable amount of seconds) only when opening the message to get a full view, while not doing it when just using the preview pane. I like having a small preview pane visible and found this feature desirable because the cursor always has some message highlighted, and I didn't wanted to show it as read unless I looked at the whole message. It seems Thunderbird has no accommodation for that. I'll disable the Mark As Read function and just get in the habit of marking them read as desired by pressing the M key.

Thank you for your help!