I have been using this add-on for years, particularly the "On disk" flag which shows me whether messages in my IMAP folder have downloaded to the local disk. This has been a super useful feature. Unfortunately, the add-on is not working with the new Thunderbird release from mid-November 2018. I really miss it because I have now no idea about the synchronization status of my several Gb of E-mail.
I wonder whether an updated version of this add-on (or at least the "On disk" flag) could be developed.