Rated 4 out of 5 stars

Not compatible with 68.4.1

This review is for a previous version of the add-on (13.3). 

Account Colors is compatible with Thunderbird 68.4.1.

Try uninstalling Account Colors, restarting Firefox twice, and then reinstalling Account Colors.

If that does not work, your Thunderbird profile may be corrupted. Try creating a new Thunderbird profile.