[Dovecot] problems with imaps

hi,

i'm using dovecot 1.2.15 with self signed certificates

using starttls on ports 110/143 works ok with thunderbird 10.0.12 (and i guess most other clients)

using imaps on port 993 works with outlook 2002.

with thunderbird 10.0.12 i can't connect to port 993 and get errors in the logs like

TLS: SSL_read() failed: error:14094412:SSL routines:SSL3_READ_BYTES:sslv3 alert bad certificate

(certificate generated by dovecot mkcert.sh)

or

TLS: SSL_read() failed: error:14094418:SSL routines:SSL3_READ_BYTES:tlsv1 alert unknown ca

(certificate generated by own openssl cmdline)

i know that a lot of information is missing, but since this looks like a more general problem i dare to ask for help before posting more config details

thx matthias