I'm accessing HTTPS web sites over an intercepting proxy. This works fine for most sites, like Google - I just add an exception and it's fine. However, on https://github.com I get the following message:
github.com uses an invalid security certificate.
The certificate is not trusted because the issuer certificate is not trusted.
(Error code: sec_error_untrusted_issuer)
There's no button to add an exception. I've tried adding an exception manually at Edit → Preferences → Advanced → Encryption → View Certificates → Servers → Add Exception..., but there's no change. I've also tried restarting the browser.
Using Firefox 21.0.