If I choose to show the zoom level the toolbar icon shows only the number 10 instead of 100 for the zoom level. It's also not aligned properly with any other toolbar icon, it looks out of place. For now I just disable the zoom level to the default icon, unfortunately the black % icon is barely readable using the new built-in Firefox dark theme. Hopefully there will be an option to change the color to white.

EDIT: The issue with 100 being cut off was was due to a font setting in my userChrome.css.