You can use the Firefox browser on Athena by clicking the ``WWW'' icon at the bottom left. Click the button only once; it may take up to a minute to start on some computers.
For most of the websites at MIT, instead of logging in, you use an ``MIT certificate''. You can get one by going to web.mit.edu/ist/cert and clicking on both ``Get'' links. This will securely store your certificate inside your Athena account. (Note that Firefox will ask you to create a master password, to encrypt your certificate, after you tell MIT your Kerberos password. You should avoid using the same password.)