The dynamic.12306.cn certificate is not valid - you should install a valid certificate

The dynamic.12306.cn certificate is not valid - you should install a valid certificate on dynamic.12306.cn.

If I try to open https://dynamic.12306.cn/otsweb/ in Firefox, Firefox will reject the connection:

dynamic.12306.cn uses an invalid security certificate.The certificate is not trusted because it was issued by an invalid CA certificate.(Error code: sec_error_inadequate_key_usage)

The only way I can get firefox to open the connection is to click on "I understand the risks" on the interstitial error page displayed by firefox and add an exception for the site.

Opening up the certificate from https://dynamic.12306.cn/otsweb/, you can see that it's key usage