Postbox

Postbox is commercial fork of Mozilla Thunderbird.

INFO:

Thunderbird Extensions Found To Work With Postbox

While a fork of Thunderbird there is no guarantee that extensions created for it will work with Postbox, it is more a case of trial and error.

The following are ones that have been found to be compatible with Postbox 4.0.3:

gContactSync

HOWTO: FIX:

Failure Of MAC OS X Contacts (Address Book) Integration

This traces back to a value not being written to the configuration file after authentication Thunderbird in System Preferences | Security and Privacy |

Config name:	ldap_2.servers.osx.description
Value:	Mac OS X Address Book

Thanks to Metaparadigma, which while not the actual solution above pointed me in the correct direction.