Firefox 48 finally includes a fix allowing checkboxes to show using GTK 3.20. They are indeed visible using the latest Numix theme, but there are still issues with the Shiki Revival and Classic themes.
This project is now abandoned, I don't have the know-how to maintain this theme in order to cope with GTK developers constantly breaking things. Sorry.
Firefox 48 finally includes a fix allowing checkboxes to show using GTK 3.20. They are indeed visible using the latest Numix theme, but there are still issues with the Shiki Revival and Classic themes.