Hi
This is the notable difference which i could find between FWSM and PIX but FWSM uses the same technology or operational method similar to the PIX.
Also the IOS versions , CLIs too are similar to PIX..
You must also use the access-list command to implement access lists that permit traffic to flow through the FWSM.
By default, access lists deny all traffic on all interfaces (deny ip any any).
This behavior differs from the PIX's default configuration, which allows traffic from higher to lower security and denies traffic from lower to higher security.
To get the FWSM to behave like the PIX, configure an access list with permit ip any any and apply it to the high-security interface
for more info do check the data sheets of both FWSM and PIX..
http://www.cisco.com/en/US/products/hw/modules/ps2706/products_data_sheet0900aecd80281886.html
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_data_sheets_list.html
regds