From a customization standpoint, one of the most attractive changes added in Windows 10 is the ability to switch between system-wide dark and light themes. But if you prefer to go dark, that setting is only as good as the apps that work in concert with it. For Firefox fans, you'll soon be able to count your browser of choice among them.
This week, Mozilla added the option for the latest Firefox Nightly build to respect your Windows 10 theme settings. So, if you choose to go with the system-wide dark theme, Firefox will automatically switch to its dark theme to match. Likewise, choosing the light theme will keep Firefox light as well. Thankfully, if you use a custom theme, Firefox won't bother to switch things up (via MSPU).
Firefox's nightly release channel is used as a development branch to test out new features, including the bugs that come with them, so it's not for anyone unwilling to put up with an unstable browser. However, now that this setting is included in the development branch, it should eventually make its way down to the main Firefox release version.
If you're curious, you can give the feature a shot now by grabbing the latest Firefox Nightly build at Mozilla.