By default the autocomplete uses the cn which is username, this changes that to displayName instead. Use dig -t srv _kerberos._tcp.example.com to get a list of ldap servers. The search base is usually dc=example,dc=com for your domain but you should narrow it down to just users to make the searches to faster.

• Setup thunderbird to login to ldap with DOMAIN\username.
• user_pref("ldap_2.servers.default.attrmap.DisplayName", "displayName");
• user_pref("ldap_2.servers.SERVERNAME.autoComplete.filterTemplate", "(|(displayName=%v*)(uid=%v*)(mail=%v*)(sn=%v*))");
• user_pref("ldap_2.servers.SERVERNAME.autoComplete.nameFormat", "[displayName]");