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.
In about:config set
security.use_mozillapkix_verification = false
To find out more about mozilla::pkix and why your firefox just got so super secure and paranoid, that it doesn’t allows you to access you own site without googling see https://wiki.mozilla.org/SecurityEngineering/Certificate_Verification. I’m only wondering why did they renamed it from insanity::pkix to mozilla::pkix – do they confess that ‘mozilla’ is slowly becoming a synonym for ‘insane’ ?-) Throwing such an error without any hint or possiblity to add an exception (as usual) is IMHO insane – but, who cares about power users today…
Update: As noted by comments, this will be not work in Firefox 33 (or later).
23 Jul 2014