Okay, stupid question, but since I don't know how your network was configured before:

Do you now have the hosts trying to traverse the PIX with the internal interface of the PIX as their default route?

Oh, and I'm going to have to second the other opinion that the PIX is about the worst firewall ever, with the possible exception of SonicWall products. The PIX CLI is awful, whereas the Cisco router CLI makes a lot of sense, IMO. I wonder if they acquired PIX from somewhere else?