I am so glad this was updated for Thunderbird, now if we could only get a version that works with the Waterfox browser which I just switched to today from Firefox 56 since I don't like Quantum and all my addons seem to work so far except for this one. I can't get any of the suggested fixes to work, even deleting the expiration date in the Theme Font Changer xpi file. Waterfox is suppose to open an independent browser add-on store so maybe that will help the issue.

Thanks for the hard work that goes into these addons, they make life noticeably less annoying.