Grey italic folders
From MozillaZine Knowledge Base
A IMAP folder will be displayed in grey italics if the folder has a \Noselect flag. Normally this means the folder can contain child folders, but not messages.  This is a IMAP protocol flag, not one of the folder flags that can be set using the Folder Flag extension.
This most commonly occurs if you use the default settings for the IMAP server directory in Tools -> Account Settings -> Server Settings -> Advanced for a Gmail IMAP account. It you set it to [Gmail] it will replace the grey italic [Gmail] folder with its child folders. You might also run into it with a UW IMAP server .
You can verify this by enabling IMAP logging and looking in the log file with a text editor for \Noselect in the same line as the folders name.
A "mailbox" is IMAP-speak for what a lot of other software calls a folder or mail folder. IMAP uses different terminology because its only meant to refer to a named object that can contain messages. It doesn't have to be able to also include other mailboxes. If the object has a \NoSelect flag it is not a mailbox  and should not be able to be subscribed.
A "dual-use name" (or "dual use mailbox") is one that can contain both messages and mailboxes. Some email clients need to be told whether an IMAP server does or does not support dual-use names. Thats why Thunderbird has a "Server supports folders that contain sub-folders and messages" checkbox in Tools -> Account Settings -> Advanced .