Review for Google Contacts by Thomas Nielsen
Rated 4 out of 5 stars
We've been using this for more than a year now, and it works like a charm. Android devices, Gmail webmail usage and Thunderbird.
Only thing missing is support for contact groups in Thunderbird itself, e.g. in Collected Emails etc. but that is hardly the plugin that must provide that feature.
To create your own collections, you must have a Mozilla Add-ons account.