Because it's still happening, also on Quantum. I still see the 5-15% CPU usage on idle. If you strace Firefox, you'll see the syscall spam, even when the browser is apparently fully idle. This consumes a lot of battery power because it prevents the CPU going to sleep, even when you're looking at a still screen in Firefox (compare to gnome-terminal as mentioned in the bug report, which stops spamming as soon as nothing moves).
My laptop lasts only half as long on battery when Firefox is on and showing a still screen.
Because it's still happening, also on Quantum. I still see the 5-15% CPU usage on idle. If you strace Firefox, you'll see the syscall spam, even when the browser is apparently fully idle. This consumes a lot of battery power because it prevents the CPU going to sleep, even when you're looking at a still screen in Firefox (compare to gnome-terminal as mentioned in the bug report, which stops spamming as soon as nothing moves).
My laptop lasts only half as long on battery when Firefox is on and showing a still screen.