[bvi]

I'm on Firefox 56. Is there a way to see if my extensions (and FT DeepDark theme) works on 57?

[sgift]

If you go to the add-on page of your extension there's a "Works with FF57+" if it works.

[NuDinNou]

It does not. But FF57 comes with a compact and dark theme, you can select it in the Customize window.