No option on the "Privacy Error" screen
It's not something you can override. The Firefox error screen is a little more informative:
The owner of feedback.appharbor.com has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.
This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate.
...
feedback.appharbor.com uses an invalid security certificate. The certificate is only valid for the following names: ssl149278.cloudflaressl.com, *.uservoice.com, uservoice.com Error code: SSL_ERROR_BAD_CERT_DOMAIN
I suggest emailing [email protected] or [email protected] to report the error.