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Why this extension is not working on this latest version of thunderbird?

Это отзыв для предыдущей версии этого дополнения (3.0.0). 

It wasn't working because it is difficult to keep up with the rate of change in Thunderbird. They keep removing features that gContactSync uses, and don't have updated documentation to help add-on developers migrate to newer versions. Version 3.1.4 should work better, but requires TB 65 or later, which is still in beta.