This article describes Thunderbird's main window, as seen in this image
The Folder pane is where your accounts and folders will appear. It will show the account name(s) specified in the account settings window, with default account first, and any additional accounts listed in the order you added them.
Within each account, special folders (Inbox, Sent, Trash) will be listed first with unique icons, while other folders will appear in alphabetical order.
Sub-folders will appear underneath the parent folder. To expand or collapse an account or parent folder, click on the arrow beside the account or parent folder, or double click on the name.
Message List pane
The Message List pane contains the list of messages in the selected folder, including details like the date/time, subject, sender, and more. To pick which details are displayed, and how wide each column is, see Message List Columns.
The Header pane will show you information about the selected message, such as the From, To, and Subject headers, as well as the date/time of the message.
Here, you can take actions on the message, such as reply, forward, archive or delete the message, mark the message as junk or as read, open the message in conversation or a new tab, view the message source, save the message, or print it.
Want to re-order or remove buttons? Click . Want a different header view? Visit the headers menu (someone better than me can fill in the lbanks), or install the compactheader add-on.
Click on the star beside a correspondent to add that person to your address book. If the correspondent is in your address book, the star will turn solid blue. Click on the blue star to edit or delete the address book entry.
The Message pane is where you see the content of the selected message. To show/hide the Message pane, press F8.
The Today pane will display information from your calendar, such as any events scheduled for today, tomorrow, and within the next 5 days. Click on the arrows at the top of the Today pane to switch to the Events and Tasks view, which will include your calendar tasks as well as events, or the Tasks view which will only include your calendar tasks, not events. To hide the Today pane, click on thebeside the arrows. To bring it back, click on in the status bar.