Rated 4 out of 5 stars

Phillipp, thank you for all your hard work in restoring this must-have add-on to its former glory.

I have found one minor bug: when I launch Thunderbird, the folder selector/identifier is inoperable and appears as a series of nested rectangles, taller than the other icons on the Mail Toolbar and with an overall length of about 5cm on my laptop.

This can easily be rectified, by right-clicking on the toolbar and selecting Customise... and then Done (or just press Enter) which results in a single normal-height rectangle, with its length reduced to 3.5cm. The add-on will now work as intended.

This review is for a previous version of the add-on (1.7.0).  This user has other reviews of this add-on.