@PhoneBoy's answer is partially correct, but it depends on the code version and Jumbo HFA you are using. These commands assume SecureXL is enabled:
R80.10 and earlier:
fw ctl set int fw_antispoofing_enabled 0
sim feature anti_spoofing off; fwaccel off; fwaccel on
R80.20 GA through Jumbo HFA Take 102: Impossible
R80.30 (kernel 2.6.18) GA through Jumbo HFA Take 75: Impossible
R80.20 Jumbo HFA Take 103+, R80.30 (kernel 3.10) GA, R80.30 (kernel 2.6.18) Jumbo HFA 76+, R80.40 GA:
fw ctl set int fw_antispoofing_enabled 0
fw ctl set int sim_anti_spoofing_enabled 0 -a
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com