Folder pane not shown
From MozillaZine Knowledge Base
The folder pane is the pane at the left. It normally lists your accounts and folders. The thread pane on the right normally lists the contents of the folder. If the folder pane is shown but is empty, see Empty folder pane.
If you don't see a folder pane it is probably because you inadvertently closed (collapsed) it (Bug 340982). To bring it back, just move your cursor to the far left side of the window until the cursor changes into a double-headed arrow (↔), and then drag the splitter bar to the right. Mozilla Suite users can just click on the "grippy" in the middle of the bar at the left edge of the window.