Screensaver ON, browsers go to SLEEP.

If you execute the lines below when the screensaver is activated / deactivated, your PC will be very quiet, enjoying that 'dark' moment, cooling down, without beer & without fan, just cool & quiet.

killall --ignore-case --quiet -s STOP  iceweasel chromium firefox firefox-esr waterfox waterfox-bin google-chrome /opt/waterfox/waterfox

killall --ignore-case --quiet -s CONT  iceweasel chromium firefox firefox-esr waterfox waterfox-bin google-chrome /opt/waterfox/waterfox

On https://unix.stackexchange.com/a/368632/270700

you can find a nice script where to insert those lines.

Just today I realised that adding "/opt/waterfox/waterfox" despite "waterfox" was already on the list does matter. So I guess, the next time, it is better to run it manually (the killall line), then try "htop" and verify they all went to sleep. If not, keep adding "noise makers" until there is no one left making noise.