It might be the cocktail of addons, but now when I have fibery running, I can hear the fan. Killing the tab is like Switching it off. On task manager, I can see the effect of fibery being open or not.
It works fine with the same document url on edge.
UPDATE: it seems to be an interaction with extension ‘midnight lizzard’ Midnight Lizard – Get this Extension for 🦊 Firefox (en-US)