Rated 3 out of 5 stars

As csmhu wrote, there is this odd extra window opened if, while IMAP, upload and download on start/exit are turned on. I had to turn them off, and manually do the upload & download.
The window is present on Win 8.1 and 10, both x64.
Three stars only, because now I could manually copy the addressbook to the IMAP folder, with no need of any add-on. I would give five gladly, if it was a flawless real sync of each record.
Console returns no errors.

This review is for a previous version of the add-on (3.14).