The pix cannot act as an http proxy server. It can use one to insure that URL requests are authenticated/authorized before sending them outbound, but it cannot perform the function itself. I believe that the url filter command can be of use, but again only for n2h2 or websense servers are supported according to the pix 6.3 docs. I have heard that other http servers may be supported as well, as cisco generalized the code that at one time was specific to websense.
Let me know if you need more help.
