The Zimbra LDAP directory provides access to the GAL, not Contacts.
Thunderbird: https://www.whitman.edu/content/wcts/support/email/zimbra/accessing/thunderbird
Access: http://www.3open.org/d/zimbra/access_gal_or_address_books_from_third_party_clients
LDAP Explorer Tool: https://ldaptool.sourceforge.net/