Review for Google Contacts by Pogle
Rated 1 out of 5 stars
This trashed my Google Contacts, sporadically putting one person's email or display name or numbers in another person's record. There's no way I can check everything manually so I'll just have to delete and start again. Fortunately I have a backup but I'll have to redo the contact groups.
Ultimately though, Thunderbird's address book is pretty basic and I don't trust it as a synchronisation source, however good the add-on.
To create your own collections, you must have a Mozilla Add-ons account.