You can set a longer time that you can display the message before it is marked as read but in Thunderbird if you open it in a tab or the message pane you have read it.

The time setting is at ..

Tools-options-Advanced-Reading and Display

No menu bar with tools showing? Press the alt key.

Sorry, that isn't correct. if I set a delay before it is marked as read, even if I explicitly open an email it doesn't get marked as "Read" until that time expires. In Outlook / Live Mail / Hotmail / Lotus Notes, etc. I can set it to show as Unread if I view it in the Preview Pane, but it gets marked as Read if I actually open it by double-clicking or pressing Enter.

Then use Outlook. This is how Thunderbird works. Thunderbird does not have a preview pane. It is the message pane where you open a message instead of opening in a tab.

I will, thanks. It looks like I am keener to use your product than you are for me to use it.

Its not my product. Maybe educate yourself and learn that this forum is staffed by Thunderbird users.

Isn't that like staffing a hospital with patients?