@istvan
Well, mine runs Void with all the latest updates and more or less recent Firefox (they've been messing with UI again and my tweaks stopped working, so I'm holding the updates off).
On idle Firefox' CPU usage is like 1.5-2%
With w3m I can squeeze an hour and a half of life even out of the two (!) decade old Thinkpad T43 😂
It's quite amazing how much more battery life you can get with old computers if you just get rid of systemd, with runit when computer is idle — it's indeed idle.