Firefox SSL – “Add Exception” button doesn’t appear

Steps to fixing the problem:

1. Close firefox;
2. Go to C:\Users\<your user>\AppData\Roaming\Mozilla\Firefox\Profiles\<some profile>
3. Delete SiteSecurityServiceState.txt

Source:

http://stackoverflow.com/questions/40126253/add-exception-to-an-invalid-ssl-certificate-website-sending-hsts-header