When a client wants to browse the internet through the PIX, the PIX is not responsible for ip address to hostname resolution, you have to sepcify the DNS server address on your end-client which will be used for name resolution. This DNS server can either be behind the PIX or outside the PIX. If you are not using DHCP then you need to statically assign IP address to clients alongwith DNS/WINS server it will use.
HTH
R/Yusuf