Create a new LDAP directory

Switch to adress book tab, click New Address Book / Add LDAP Address Book:

Choose a name, host is DavMail host (usually localhost), Base DN is ou=people, adjust port to your settings and set user to email address or Active Directory account name for older Exchange servers. Leave other options to default values.

You may have to use uid= prefix as username to make it look like a valid dn: uid=account

Note that password will be requested on first search:

You may also want to use this directory for mail completion in compose window. Check Directory Server and choose directory:

Proceed to Thunderbird Carddav setup