From MozillaZine Knowledge Base

This article applies to Thunderbird and Mozilla Suite.

Normally the folder pane, showing all your accounts and their various folders, should be displayed on the left-hand side of the main window. If for some reason you do not see the folder pane there, it is probably because you inadvertently closed it. To bring it back, just move your cursor to the far left side of the window until it changes into a double-headed arrow (↔), and then drag the bar to the right. Mozilla Suite users can pop open the folder pane by simply clicking on the "grippy" in the middle of the bar at the left edge of the window.

For Thunderbird users, the grippies extension will give you the same grippies found in Mozilla Suite, so that you can toggle open/shut the folder pane, preview pane, etc. at a single click.