If your add-on is no longer working, you'll need to downgrade to the previous version:
1- Remove (uninstall) the Theme Font& Size Changer addon
2- Go to this page and download/install version 56.0
https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/versions/
3- To prevent the add-on from updating itself to the latest version, you'll need to disable it (note: this will also disable auto-updates for all other add-ons). Follow these steps here: https://blog.mozilla.org/addons/how-to-turn-off-add-on-updates/