Firewall – 1

I have installed Firewall -1 on an NT4 svr (SP5) …. When users browse to certain sites such www.amazon.co.uk or www.hp.com/uk the browsers just time out and report that they cannot find the page requested… But i can browse to the pages directly from the Firewall …. I have checked all the wkstns G/W settings etc.. they are all only runnign TCP/IP and there appears to be no obvious problems… URGENT !! I would be very grateful of some Ideas .. Thanks