For all those complaining about the latest version not working (aside from the developer's action and more importantly, the handcuffs imposed by Mozilla on FF 57+ which kills and prevents many add-ons from EVER working with Web Extensions) - stay with Theme Font & Size Changer version 56, available here: https://addons.mozilla.org/en-GB/firefox/addon/theme-font-size-changer/versions/ and stay with Firefox version 56.

You can lock the current version of any add-on via the Add-on Manager: click on the "More" link next to an add-on and change "Automatic Updates" to "Off".
Firefox still alerts if an update is available but doesn't automatically update it, giving you the opportunity to check the changes made/reviews BEFORE any changes get made.