Rated 5 out of 5 stars

Installing the previous version (3.5) does seem to fix the problem. Though, when starting Thunderbird, there is now a slight delay (less than a second) before the standard button icons are replaced with the Phoenity ones. No big deal, it's acceptable. Thanks for the fix! Continue the good work... Maybe more custom sets of icons?

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

Rated 5 out of 5 stars

Terrific add-on... while it lasted! It really dressed up Thunderbird's toolbar, but with the latest update it shuts off every time Thunderbird is closed. Please fix it! The biggest drawback of using Thunderbird is it's bland, clinical look. Add on' s like this one are remedying that. Thanks!

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.