Core issue
There is no way to automatically check the fingerprint on the PIX Firewall with software version 6.x. This is due to the presence of Cisco bug ID CSCec50476.
Resolution
To check the full public key on the PIX, issue the show ca mypubkey rsa command. After the full public key is cached on the Secure Shell (SSH) client, manually compare it with the key shown by the PIX.