Rated 5 out of 5 stars

Update 09-oct-2019 for v42.4: Just noticed that my addressbook is back -- thanks for the fix!
Cardbook now earns it's 5 stars again.
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Used to be a very good plugin. But when upgrading Thunderbird from v60 to v68 (and updating the plugin accordingly), the entire addressbook vanished, and I find no trace of its database file. [based on v41.6]

This review is for a previous version of the add-on (41.6).