Folders and views
From MozillaZine Knowledge Base
This article describes some ways to customize the main window layout, the folder pane, and what messages are displayed in a folder listing (aka thread pane). It tries to ignore mainly cosmetic changes.
Advanced Folder Views
Thunderbird supports several "advanced folder views" that change the layout and contents of the folder pane. It defaults to "Unified Folders", which creates several virtual folders. One of them is labeled "Inbox" and has child folders that are each accounts inbox folder. It creates similar virtual folders for Drafts, Templates, Sent, Junk and Trash.
A virtual folder (Mozilla Messaging calls them a " saved search" ) is a way to re-organize and manipulate messages from one or more folders/accounts as if they were really stored in that folder, but it doesn't change where the messages are physically stored. You can use the ◄►arrows next to the name to switch to "Recent Folders", "Favorite Folders", "Unread Folders", and "All Folders". "All Folders" is similar to the folder pane in Thunderbird 2.* - it doesn't modify anything.
The reason they're called advanced folder views rather than virtual folders is because you can't change the search criteria for the virtual folders, and they may include several of them. The closest you can do is choose favorites by right clicking on a folder, and selecting Favorite Folder. Those folders will be listed if you use the the ◄►arrows to select "Favorite Folders" . You can also use View -> Folders to select a advanced folder view.
The Additional Folder Views add-on adds another tree view (Unread, Favorite and Recent) above the existing one. However, it does not work (even if you disable version checking) with Thunderbird 3 or later.
A Global_Inbox is a optional feature that stores mail from multiple POP accounts in a single Inbox in Local Folders. You can have all, some or none of your POP accounts use the Global Inbox. Each account that does not use the Global Inbox will have its own set of folders, including its own Inbox, displayed in the folders pane. Accounts that use the Global Inbox will not have their own folders displayed in the folders pane; instead, all you will see is the single set of folders in Local Folders.
The article mentions several other alternatives that would let you also support IMAP accounts.
There are two types of views. One is used by View -> Layout to specify the overall layout of the main window such as whether you want the folder pane on the left, and the folder listing on the right above the optional optional message pane or do you want all three side by side. The MoreLayoutsforThunderbird add-on adds a Wide Thread View and Stacked View. It will work with Thunderbird 3.* and 3.1.* if you bypass the version checking.
Its also used to refer to different ways to display (view) the contents of the current folder. Right click on the main toolbar, select customize, drag and drop the "View" control to the toolbar, and press Done to add that control. It has several predefined choices such as All, Unread, Not Deleted, Tags, Custom Views, Save View as a Folder, and Customize. All displays everything as is. Not Deleted hides any messages that are not deleted. This is useful in a IMAP account where you can have deleted messages still listed in the folder pane (with a line drawn through them) rather than being moved to the trash folder. Tags lets you select you a single tag. Thunderbird will only display messages in that folder that have that tag.
"Save View as a Folder" saves the view as a virtual folder (saved search). "Customize" lets you you create custom views. It has predefined custom views for People I know, Recent mail, Last 5 days, Not Junk, and Has Attachments. The "Message View Setup" window that it uses has the same search criteria as when you create a virtual folder except it doesn't let you specify different folders/accounts. The main limitation of a view is that its folder specific, there is no way to integrate it with the folder pane so that it applies to whatever folder you are looking at.
Remove or restore missing folder pane
FolderSelection A popup window for selecting any folder when the folder pane is hidden
See Show Inbox when starting Thunderbird. You can also use command line arguments to specify a different folder.
IMAP Folder hierarchy
Folder pane properties
 Account Colors
==Additional columns in folder listing]