https://wccftech.com/firefox-goes-dark-windows-10/

Mozilla has just released a new Firefox Nightly Build, which will respect your Windows 10 Dark or Light setting.

It may have been a few years since Dark Mode was introduced to the Windows world but the adoption by apps – and even Microsoft in several cases – has been slow. Even the File Explorer has just started to comply with the system settings, and that too for Insiders only. Regardless of a rather broken implementation, the ability to go system-wide dark (or light) has potentially remained one of the most favorite customization features.