firefox-launcher starts/stops/suspends multiple firefoxes.
go get -u github.com/susurri/firefox-launcher/cmd/firefox-launcher
Run firefox-launcher and type help at the prompt.
suspend firefox when it is not an active window.
An EWMH/NetWM compatible X Window Manager.
It has been tested on Gentoo Linux running Cinnamon desktop.
It may work on the other Linux distributions and window managers.
The software is available as open source under the terms of the MIT License.