Rated 3 out of 5 stars

Good work, but there are two missing features:
1. Left-click on the icon does not restore the windows (unlike all the KDE applications).
2. It does not inform me that the "newest" messages (marked by stars in the Thunderbird) has come. I have a great need of this feature, since I often delay reading some new messages (which may be important, but not urgent), but I still need to be notified about newer messages (which may be more urgent). Since I am not sitting near the computer permanently, I may skip the beep or pop-up notification, but I have to know about it just by throwing a glance at the display, without keeping in mind the initial number of new messages. It could be realized, for example, by changing the color of the icon, once new mail arrives. (This feature existed in the "New Mail Icon" add-on, which seems to be currently unsupported and does not work with TB3.)

This review is for a previous version of the add-on (0.2.3).