From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
Thunderbird can list an empty non-existent folder in the folder pane. If this occurs exit Thunderbird, back up your profile and:
- Delete the "panacea.dat" file in your profile folder. It's a mail folder cache.
- Delete the ".msf" (mail summary file) file for that folder in the account's directory in your profile. Thunderbird believes a folder exists as long as it sees a file with the folder's name and an .msf file extension. If it's easier you could delete all .msf files. The only damage you could do is lose a Saved Search folder.
- Delete "XUL.mfl" (if it exists). It's a cache of user interface data. (XUL.mfl is replaced with xul.mfasl under Linux and XUL FastLoad File under OS X)
- If you're using an IMAP account, enable IMAP logging and look in the log file to see whether the IMAP server returns that folder name. That helps identify whether the phantom folder is due to the client or the server (ISP). If its a UNIX IMAP server you might also check whether a ".mailboxlist" and/or ".mlbxlsttmp" file in your root directory mentions the phantom folder.
- If you're using a News account, look in the newsrc file for the NEWS server for the newsgroup name. Delete any duplicate lines.
Thunderbird will create a new panacea.dat, XUL.mfl, and *.msf file(s) if they're missing the next time you run it.
If none of this helps, use the Profile Manager to create a new profile and then migrate your folders, address books and settings to it. Don't just copy all of the files from your old profile to it or you'll migrate the phantom folder too.