So you want a customer header in TB...

create/edit user.js file:

user_pref("mail.identity.id1.headers","pgp1,pgp2");
user_pref("mail.identity.id1.header.pgp1","X-PGP-Fingerprint: CABE 26F2 DF3A 7457 9B9C 74A6 EAA2 62C8 B0A2 B90F");
user_pref("mail.identity.id1.header.pgp2","X-PGP-Key-ID: 0xB0A2B90F");

restart. pretty simple. id1 being the first account, id2 the second, etc.

For more detailed info, check out https://bugzilla.mozilla.org/show_bug.cgi?id=61520 The most recent stuff is at the bottom