Review for gContactSync by Dahu l\'Arthropode
Rated 5 out of 5 stars
Any other module I tried eventually corrupted data on one side or the other.
This is the first acceptable solution I found to synchronise Thunderbird and Google (and, hence, my iPhone) in a reliable way.
The next release taking advantage of Google contacts improvements sounds promising.
To create your own collections, you must have a Mozilla Add-ons account.