Rated 5 out of 5 stars

Philippe, works OK now due to the update. Thank you for your prompt attention in this matter.

This review is for a previous version of the add-on (21.1). 

Rated 1 out of 5 stars

Mac Sierra 10.12.6 - Thunderbird 52.2.1 (64 bit)
Disabled then Enabled Add-ons until I found the culprit which was hiding the red, orange and green menu bar buttons

This review is for a previous version of the add-on (20.7). 

this is fixed in the current 21.1... sorry...