Firefox supports Kerberos, but, you have to tell it which domains it trusts to send the Kerberos tokens too.
1.Open Firefox
2.In the address bar type: about:config
3.Firefox3.x and later requires you to agree that you will proceed with caution.
4.After the config page loads, in the filter box type: network.negotiate-auth
5.Modify network.negotiate-auth.trusted-uris by double clicking the row and enter yourdomain.com
6.Multiple domains can be added by comma delimiting them such as yourdomain.com, yourotherdomain.com
Also, if you have already tried to authenticate through the stack in your current Firefox session, you will need to restart Firefox for this to work.