Google has started to provied
new APIs to manage contact groups in Google Contacts.
Google Contacts 0.2.10, which is available for download now, maps Google's contact groups to Thunderbird's mail lists. As previous versions updates cards/contacts interactively, lists/groups are synchronized.
Moreover, you can store Thunderbird specific card items to Google a contact. This is achieved by using new Google APIs for extended properties. These properties are not shown in Gmail but you can see them everywhere if you access them by Thunderbird with the extension.
The new APIs includes another interesting features: support for contact photos. This is not implemented yet in the extension because TB address book card does not include photos.
This version includes many changes from the previous versions. But it is not tested enough and it may involve serious problems. So, don't forget to backup your Google contacts. And please let me know if you find any problems.