Take a look at this example configuration at the PIX example.
HTTP and FTP Proxy Caching Using a Cisco Cache Engine 550 and a PIX Firewall:
http://www.cisco.com/en/US/products/hw/contnetw/ps546/products_configuration_example09186a008009460a.shtml
Here's is an explaination for the fixup http but I don't think this has something to do with your configuration.
fixup protocol http
The fixup protocol http command sets the port for Hypertext Transfer Protocol (HTTP) traffic application inspection. The default port for HTTP is 80.
Use the port option to change the default port assignments from 80. Use the port-port option to apply HTTP application inspection to a range of port numbers.
Note The no fixup protocol http command statement also disables the filter url command.
HTTP inspection performs several functions:
URL logging of GET messages
URL screening through N2H2 or Websense
Java and ActiveX filtering
The latter two features must be configured in conjuction with the filter command.
SEE:http://www.cisco.com/en/US/products/sw/secursw/ps2120/products_command_reference_chapter09186a00801727a8.html#wp1067379
sincerly
Patrick