this topic has been discussed before and always seemed to run into a dead end.
Thunderbird allows the encryption of account-login-name and password.
All other info is still available unencrypted in the files in the folder profile. This includes my email-addresses and mail-providers, address book and at least some of the mail-headers and subjects in the folders.
The latter can be resolved by (securely) deleting all the mailboxes (*.msf) or better still the folders .../ImapMail and .../Mail after shutting down thunderbird. These folders are then recreated once thunderbird is started again.
Obviously the other infos cannot be deleted. That means that if someone finds/steals my USB-drive he will have lots of information he could use to try to access my mail or send mails in my name or hassle my contacts etc.
IHMO this is serious and means that thunderbird cannot be used on a USB-drive.
Encryption as with truecrypt is not an option because it does work unless you have admin-rights. And these you will usually _not_ have if you really go portable.
The only real solution I can imagine would be to have thunderbird itself encrypt the data.
Is there some other mail-client around that supports IMAP and multiple accounts and locally encrypts the data?