Certificates in Zimbra

[Stas43]

Good afternoon!

Question according to certificates in Zimbra 8.5.1_GA_3056....
Everything works. There are no problems as if.
The certificate of startssl for a period of 3 years is set.
SUDDENLY!!!! There is Firefox (my OS — Ubuntu) decided that it is not entrusted certificate and ceased to start up connection through HTTPS. At the same time Zimbra desktop clients work perfectly. Cromium, IE — work correctly

Changeover of the certificate in Zimbra isn't full automated. Right?

I want:
1. to switch Zimbra from https to http
2. to leave startssl the certificate only for Zimbra (3 years !!)
3. to set nginx and to redirect 443 on the 80th Zimbra port. In addition to set letsencrypt certificates in nginx — it is completely and it is easy automated.

Thanks. Also excuse for Google translator:)