c'est possible en modifiant 2 variables:
mail.identity.id1.doBcc user set Boolean false
mail.identity.id1.doBccList user set string
[http://sillydog.org/forum/sdt_12639.php

http://kb.mozillazine.org/BCC

You can configure an account using Tools -> Account Settings -> Copies & Folders -> "When sending message, automatically" to always BCC one or more email addresses sent using the identity. Thunderbird will store that information using the mail.identity.idN.doBcc and mail.identity.idN.doBccList preferences, where N is the identity id. That will display the Bcc: field(s) at the top when composing the message. If you delete the empty To: field and press return it will (temporarily) change the default to Bcc:. That is the only known way to change the default from To: to Bcc:.