In the older version of Cisco Pix firewalls the serial failover cable on the PIX Firewall is used for failover communication,this cable is limited to six feet. This is the only size serial failover cable that Cisco makes. As such, this is the only length supported.
In the latest version firewalls i.e. Cisco ASA firewalls use the LAN based failover so for this failover the the firewalls should have Layer 2 connectivity through LAN interface.