my solution:
1. get an account on scheduleworld.com
2. install the scheduleworld-plugin for thunderbird
3. configure roundcube with the scheduleworld-ldap-server
so, you can syncronise TB with scheduleworld and automatically you will get all contacts in roundcube (by ldap contacts).
one problem: if you have more than one email for a contact, autocomplete in roundcube shows only the first one.