lets see there is usually a tab opened that has some sort of welcome message when you first start the application. SO are you talking about a tab that you need to close perhaps and once closed you see again after the program updates in about a month

There is no TAB. Just te welcome message in the right hand pane. Clicking on any mailbox does not show any messages.

Do you mean something like this, where the purple part is your welcome message?

If that is the case you click on any message listed in the middle mane and the message content replaces the welcome. It is essentially a placeholder until you select an actual message. It also offers the developers a way to pass information along without asking you to provide information so they can email you or contact you on social media. Like advise if there is a data breach or serious security flaw in your version of Thunderbird. It is mostly however a static page extolling the benefits of Thunderbird and asking for money. THe actual page displayed is control in setting as the home page.