Yesterday I installed FF31ESR and today I ran into the problem by opening a HTTPS link of a demo server with a self-signed SSL certificate.
Some search on google gave the following help ( which works fine ):
If you are trying to access site with self-signed certificate with Firefox 31 (or later) and get Issuer certificate is invalid error (sec_error_ca_cert_invalid), you have to disable new mozilla::pkix certificate verification.
> Open a new tab and enter about:config
> Then scroll down until “security.use_mozillapkix_verification = true”
> Double click this entry to set this value to false
After this the confirmation is displayed again 🙂