Contacts
A lot of people has a google account, and google contacts sotored. I installed add-on to sync thunderbird contacts with google contacts, but there are a lot of mistakes and errors.
I think that sync shouldn't be an add-on. It should be a thunderbird characteristic. And well developed. Also calendar, etc.
Opaite Mbohovái (3)
Contacts can be synced using the built-in CardDAV: in Address Book, File/New/CardDAV Address Book... enter the gmail address...contacts are autodetected...enter the account password in the OAuth window to allow access (cookies must be accepted in TB Preferences).
For calendars, File/New/Calendar...On the Network...enter the gmail address...calendars are autodetected...enter the account password in the OAuth window to allow access.
categories contacts doesn't sync with label google contacts. sometimes there is error 400, and i have to delete google account in Card-dav, and re-sign in again.
Try CardBook if you need to sync categories (groups).
https://addons.thunderbird.net/en-us/thunderbird/addon/cardbook/