Ah ok I know what option you are talking about. On
this site it says that Thunderbird "supports GSSAPI, Kerberos, CRAM_MD5, DIGEST-MD5, NTLM, and APOP. NTLM is also called Secure Password Authentication (SPA) or Windows Integrated Login." methods for Secure Authentication.
And no Zimbra doesn't seem to support it... at least my system doesn't. There are a few other threads about this on the forum.
This one seems to have the most detailed discussion that I found:
ssl and secure authentication in thunderbird
This one might have a way to enable it:
Secure MTA connection with authentication
Basically the only difference that Secure Authentication would make is you could enable Secure Authentication and then disable SSL, so the authentication would be secured but mailbox access would be insecure. As long as you have enabled SSL your entire connection is already secure.