Review for Google Contacts by HansSietsma
Rated 5 out of 5 stars
Works great!
We even use this to share contacts between office associates. Just make a separate contactsbook for the contacts you want to share, make a separate account on Google and install this add-on on every machine. Works better for us than the SyncKolab add-on because it can be set to send and receive updates automatically.
To create your own collections, you must have a Mozilla Add-ons account.