I am providing web hosting to people and recently I installed CISCO PIX 501 in other server. But I am having trouble with NAT.

I am using firewall like this (BY NAT)

Server - Firewall - PUBLIC
Private_IP-Firewall-Public_IP or Public_IP_A-Firewall-Public_IP_B

This make trouble with Cpanel/WHM. I have to edit DNS FILES Everytime when I create some account with Domain. I don't know what to do? Is there anyway I can use firewall like

Public_IP_A-Firewall-Public_IP_A ?

Or is there any suggestions for using CPANEL/WHM with Firewall CISCO PIX 501?

Yes, many people use hardware firewalls without NAT. The firewall can act a router, or even a Layer 2 bridge. I know that OpenBSD can be configured to do IP firewalling even as a layer 2 device.