we're upgrading our 6509 720 SUP IOS, which is 3.1 Hw 8.1(3) Fw, to a 12.2(33)SXI1 code. then we're to upgrade the attached FWSM to a 4.0 code.
the question is "a FWSM with 4.0 code should work on a 6509 720 SUP with 3.1 Hw 8.1(3) Fw"?
You'll need to check the IOS version running on the SUP, in addition to SUP model. Here is the 6500 requirements for running FWSM 4.x:
Hope that helps.