Version 46.5 works with Firefox ESR 52.5.3

Download the file, drag it onto the addons screen, confirm install prompt

https://addons.mozilla.org/firefox/downloads/file/438176/type:attachment/theme_font_size_changer-46.5-tb+fx+sm-mac.xpi?src=version-history

Make sure you have disabled automatic updates for the addon (double click it at "Add-ons Manager" screen, scroll down, select "Off" for "Automatic Updates")