The changes that you make take effect immediately. You can check the option - close message window/tab on move or delete' to close tabs or message windows automatically if the loaded email is moved or deleted.

The fourth option determines how move or delete operations interfere with open emails. When you select a new email, the old is unloaded and the new message is displayed. An existing message window - Opens a maximum of one window for all emails.

Multiple windows are created, one for each email that you open this way.

A new message window - Opens messages in their own window.

Tabs get reloaded when Thunderbird is started the next time, provided that it has not been closed before.

A new tab - This is the default option emails are loaded in tabs, with each email being loaded in its own tab.

It has four options, of which three determine how Thunderbird opens messages that you select: There you find the preference 'open messages in'.

While the General tab is active, scroll down to the Reading & Display section.

You can display it temporarily with a tap on the Alt-Key on the keyboard. Thunderbird may not display the menubar at the top by default.

Open the Thunderbird email client, if you have not already.

Here is how you configure Thunderbird to open emails in separate message windows or a single window. If you prefer to launch multiple messages in Thunderbird in their own window, you may configure the email client to do just that. Downside to using a single window is that you can only display one email at a time using windows. Some Thunderbird users may prefer a single window, as it keeps things neat and tidy.