Good news — from Firefox 87 this is now enabled by default, without the need to set a configuration flag! You can find it in the inspector tab:
- Choosing the sun button simulates light mode;
- Choosing the moon button simulates dark mode;
- With no button selected, your operating system default will be used, as per normal.
In some older versions of Firefox, this feature existed behind a flag. To test on one of those versions, you can thus enable this by going to about:config, and setting the devtools.inspector.color-scheme-simulation.enabled property to true. Once that’s done, you’ll find the controls for it in the same place as in the modern devtools, but as a single paintbrush icon rather than today’s sun/moon toggle buttons.