Microsoft has been adding the dark mode to apps across Windows 10 for a while now, but Mozilla is now jumping on board too. With the latest Firefox Nightly build, the web browser's light and dark themes will match your default Windows 10 setting.
— Firefox Nightly 🔥 (@FirefoxNightly) July 10, 2018
As of current, this option won't impact any custom Firefox themes, and you can tweak it by heading to Windows 10 Settings>Personalization>Colors and then "Choose your default app mode." You can grab up the latest Nightly build from this link here, but please be aware that this is a pre-release and might be unstable.
If all goes as planned, this same build and feature might roll out to all Firefox users in the next few weeks. It is pretty nifty, but still odd to see that a third-party developer has managed to enable this before Microsoft did in Edge. Anyway, let us know your thoughts on this in the comments below!