Get certificate error trying to access yahoo mail, and there is no exception to bypass the error.

I am getting a certificate error trying to access https://mail.yahoo.com. Unlike most other certificate errors, there is no option for "I understand the risks" to add an exception for the page, just the following: mail.yahoo.com uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. (Error code: sec_error_unknown_issuer) I tried setting the proxy to "no proxy" under Advanced settings, as well as clearing all cookies and other cache history. Did not resolve the problem. I cannot access my email.