Rated 5 out of 5 stars

CardBook is the missing link to finally tie TB straight in with cloud and phone, fantastic!!
* Reliable syncing to NextCloud: While inverseSOGO gave me lots of trouble, CardBook worked out of the box.
* Standards compliant: No information is lost when syncing with your phone. In contrast, the standard AB has a second address line which is ignored upon sync.
* Convenient: Full text search, groups are accessible with one click and map into android [in DavDroid set per-card-categorie]
* Allows calling Skype without Telify AddOn [under preferences-internet communication add Phone Code:skype Protocol:skype]
* Make sure to configure in Thunderbird preferences, Display tab, uncheck the last option : "Show only display name for people in my address book". Otherwise it was very laggy for me [see SOLVED - 21.4 and 21.5 makes Thunderbird extremely laggy]
* Fantastic support, Philippe is very responsive!

This review is for a previous version of the add-on (21.5).  This user has a previous review of this add-on.