Rated 5 out of 5 stars

I avoided updating Thunderbird past 52 because I hated the flat, uniform looking icons... it just makes it harder to quickly click the right one without thinking about it and hunting through them. I was forced to update though finally to v91 because of Gmail's changes. When I found these icons though, it was the perfect fix. Yes, the newest Tbird update turns the icons off whenever you start Tbird, but just shutting it off and back on quickly fixes it. The folder sort extension I use is still working fine, Manually Sort Folders, so I don't have that problem others have mentioned with it.

In case it helps, the extensions I have are:
Manually sort folders
Open Google Calendar
Phoenity Icons

I am using a theme, but even when using a default theme, the icons don't show on a Thunderbird restart until you turn off the extension and then back on again.

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.