Rated 5 out of 5 stars

--- update ---
Got immediate response from the developer on this review withe explanation about my errors.
Support can be found at: https://cardbook.icu/forum/ mentioned here (I'm sorry I overlooked that one going straight tho the start page https://gitlab.com/CardBook/CardBook mentioned in the add-on in Thunderbird itself).
So based on the response and support found at the forum I give it a big plus now!

--- Original review ---
Looks nice but makes a mess of all my contacts, removes birthdays and shows Thunderbird address books double (I don't know why). Since I can find no support or help here nor on https://gitlab.com/CardBook/CardBook I tried the following to resolve this:
1) Removed the CardBook add-on from thunderbird;
2) Restarted thunderbird;
3) Deleted all my Google contacts online;
4) Deleted al my contacts in Thunderbird;
5) Reimported my thunderbird contacts and address books;
6) reinstalled this add-on, hoping it will ask for my address books again.

Now guess what?
* All my contacts and address books still show double in CardBook in Thunderbird! No not in the address book of Thunderbird itself only in CardBook.
* And Thunderbird now locks up because it asks if I want to keep or delete a contact because it is not in Google anymore. Which IS a nice feature wasn't is so that CardBook is asking me the same question for EVERY SINGLE contact and not for the whole bunch of - in my case - 5145 contacts at once!

So with no explanation while installing, no support page available, removing birthdays from my calendar, no way to handle a large quantity of contacts and messing contacts and address books up in a undefinable way this add-on gets a zero for now.

I hope it will be better and more clear in the future because I would still like to use it after all... Thanks!

This review is for a previous version of the add-on (56.3). 

to let you know : all of this stuff is caused by Google servers... when you create a new event, a new contact or a new category with a unique ID, Google accepts these new objects then computes on them and attributes them a new fresh id... So when you sync again, all of these appear to new on the server and the ones you created are considered to be deleted.... this is not easy to handle... with Carddav servers, Apple, etc... you don't have such a mess...

What you have to do is only recreate a new Google address book in CardBook (And delete the old one in CardBook). Don't delete contacts directly. If this does not solve your issue, email me directly to cardbook (at) cardbook.icu

For the birthdays, everytime you change the event name, CardBook will create new events (as IDs are not reliable, CardBook search for names)...