The error is (Error code: sec_error_ca_cert_invalid)
Friday, August 1, 2014
Firefox users unable to connect to self-signed or Domino-based SSL
Firefox 31 has introduced new SSL library (activated by default) that generate SSL error when user attemps to access a Domino server with SSL self signed certificate.
IBM in this technote explain some workaround to this issue and recommend the use of Firefox ESR.
Firefox ESR in last days was updated to version 31 (and also use this SSL library) but the 24.7 is still available because the support will end with 24.8 next September.