I'm having trouble using it on Ubuntu Linux.
On does not work properly after Off.

instead of addon,
~$ sed -i 's/privacy.firstparty.isolate",\ true/privacy.firstparty.isolate",\ true/g' ~/.mozilla/firefox/default-release/prefs.js

This allowed me to turn isolate on and off in the terminal.
But I would have been happier if this addon had worked properly.