Resolution
You have different steps to resolve the issue:
Workaround for Firefox and Thunderbird: Disable DHE ciphers in the Zimbra Collaboration Server
Disabling the insecure ciphers, will make Firefox happy again:
zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_128_CBC_SHA zmprov mcf +zimbraSSLExcludeCipherSuites TLS_DHE_RSA_WITH_AES_256_CBC_SHA zmmailboxdctl restart
