Very simple and just works by enabling disabling Firefox proxy settings. There is one small issue though, proxy switches on automatically when browser restarts even if it was disabled.

Ben: did you follow the directions in the add-on description

Me: Yes, I did try that, still it turns on (the button becomes green) after restart. I would expected it remember the last state and restore on restart.

I agree with @15016291. When using a proxy software like Shadowsocks in Global Mode, I can't just click the "Turn off proxy" button to stop my browser connecting to the proxy server.

Works as said. Would be great if it switched between Off/On and not System/Manual, or if it gave an option to choose the two states.