I woke up to find my laptop fan was spinning. Presumably it had been spinning all night!
Why? Because after logging in last night, Firefox had upgraded itself, and loaded this page:
https://www.mozilla.org/en-US/firefox/102.0a2/whatsnew/
That page has an animated SVG (class "mandala") and this had been animating all night!
Two things could have saved energy in this situation:
1. When Firefox updates, the "whatsnew" page it opens should not show an animation.
2. When the browser is not focused, and/or when the user goes idle, animations should stop animating.