Firefox seems to keep my laptop awake, even if there’s no video playing…

You can verify if an application will keep your screen awake in macOS with the Activity Monitor app, in the ‘Energy’ tab.

The fix was to go to about:config in Firefox, dismiss the warning, search for dom.screenwakelock.enabled, and set that to false with the ‘switcheroo’ or reverse icon looking button.

When a video is actually playing, Activity Monitor shows that Firefox will keep it awake, but as soon as the video has stopped playing, it will switch back to ‘No’ in the column for ‘Preventing Sleep’

That’s all