Rated 4 out of 5 stars

Love this add-on, but indeed in the latest version of Thunderbird (91.10.0) it no longer works correctly. I should turn the add-on off and then turn it each time I open Thunderbird.

After some additional testing, I have to conclude that this add-on now causes problems for the following add-on: Manually sort folders.

So for now I am disabling this one until it is fixed.

This review is for a previous version of the add-on (3.5). 

Sorry for the inconvenience, there are a lot of reports like yours so I backed out Phoenity Icons 3.6 and went back to 3.5.
To install Phoenity Icons 3.5 go to the add-on page in the Add-ons Manager, click the Homepage link and install it again from there.

I can't reproduce it here so I do not yet know what causes it.