The folder classes all implement the nsIMsgFolder interface. There are currently three folder classes - nsLocalMailFolder, nsImapMailFolder, and nsNewsFolder. They all inherit from nsMsgDBFolder, which implements a lot of the core functionality. Rss feeds are implemented by nsLocalMailFolder.
Each line in the folder pane corresponds to a folder object.
Folders have a number of interesting attributes:
- getDatabase ( how to get a database of the messages in the folder.)
- updateFolder (gets new messages for that folder, if applicable, e.g., pop3 inboxes, imap folders, news folders, rss folders)