I'm using a speedtouch USB ADSL modem for connecting to my ISP, but I am running into problems with my network. The spare PC that I've assigned to firewall/gateway duty can get to every site I've tried, but the PC's that are using it as a gateway can't reach some sites.

Some of the sites giving me trouble are bt.com, penny-arcade.com, mail.yahoo.com, and uptime.netcraft.com. I can ping these sites from the attached desktops, but not retrieve data from them.

The firewall has ip_forward enabled, and during testing, the firewall is completely open, just NAT masquerading. With my normal firewall up, I get the same results, some websites accessible but not all. Where should I be looking?

If you can ping the site from the command line, you might want to look at changing your DNS configuration. Sounds like you have perfect connectivity, just a few configurations might need to be in order.

Also, are you using your firewall as a PROXY?
Is your firewall changing the port settings used? i.e, if bt.com wants to use port 80, is your firewall only accepting port 81? If this is the case, it could answer why only some sites have issues.