If this PIX is FO, you may use it in different network but not by itself, it needs to work in conjunction with other same model PIX for the FO pix to work properly.
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps5708/ps5709/ps2030/product_data_sheet09186a00800b0d85.html
Quote from link FO section.
Designed for use in conjunction with a similar Cisco PIX Security Appliance model that has an Unrestricted license, providing a cost-effective, Active/Standby high-availability solution.
⢠Provides the same capabilities as the Unrestricted license, except the Failover license does not support Active/Active failover. Requires presence of similar Cisco PIX Security Appliance model with an Unrestricted license to operate properly
Rgds
Jorge
Jorge Rodriguez