You can always get to cPanel through http://www.yoursite.com/cpanel. Just be aware that SSL is not being used in this case, and any sensitive data will not be protected (encrypted) during transmission, so be careful about entering or displaying passwords and other sensitive data.

Does it work with FF2? Does FF3 permit you to ignore SSL certificate warnings? IE is notorious for trying to prevent access to cPanel because the SSL certificates are for SERVER.lunarpages.com and the login is to login.SERVER.lunarpages.com. Maybe the same thing is happening here? If not, maybe the certificate has  expired and you should drop a line to support@lunarpages.com.