__IMPORTANT NOTE__: Thunderbird won't show contacts in LDAP address book directly, but it works when you starting typing email address in recipient field while composing email.
Here we take Thunderbird 5.0 for example. Steps:
* Click `Address Book` in main Thunderbird window.
* In Address Book window, click menu `File -> New -> LDAP Directory`.
* In tab `General`:
** `Name`: use whatever name you like. e.g. Global LDAP Address Book.
** `Hostname`: IP address or hostname of your iRedMail server.
** `Base DN`: Base dn of your domain in LDAP directory. Normally, it's `domainName=domain.ltd,o=domains,dc=xxx,dc=xxx`. For example, `domainName=example.com,o=domains,dc=iredmail,dc=org`.
** `Port Number`: 389. __Note__: If you prefer to connect to OpenLDAP server over SSL, please check the `Use secure connection (SSL)` under same tab.
** `Bind DN`: It's full LDAP dn of your mail account. Normally, it's `mail=user@domain.ltd,ou=Users,domainName=domain.ltd,o=domains,dc=xxx,dc=xxx`. For example: `mail=john@example.com,ou=Users,domainName=example.com,o=domains,dc=iredmail,dc=org`.
* In tab `Advanced`:
** `Don't return more than xxx results`: depends on how many accounts stored in same domain, you may want to increase or descrease it. Default value in 100.
* Now switch to tab `Offline`, click button `Download Now` to test your settings. It will prompt to input password for this LDAP server, just type password of your mail account. With correct bind dn and password, it will display `Replicated succeeded`.