ISA and Client borwser

I’ve a MS Small Business Server 2000 which has ISA 2000 in it. I’ve installed Firewall Client in all client PCs. I’m able to surf the internet only thru IE and not with any other browser like Netscape or Opera. The message I get is “This browser doesn’t support login authentication …..”

Pl. suggest me as to how to configure ISA so that all (Client) browsers are supported.