This issue occurs due to the presence of Cisco bug ID CSCsc44193.
This issue occurs when the fixup protocol FTP is enabled. If the FTP server sends a 220 or 230 reply that spans more than one packet, the PIX Firewall can drop the 220 or 230 reply packets that arrive after the initial 220 or 230 reply.
Also, if the FTP server sends a 226 message before any of the FTP data packets arrive, the PIX can drop the 226 message.
As a workaround, perform either of these steps:
Disable the FTP fixup.
Download and upgrade the software version to 6.3(5.105) or the latest version available.