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.

You can enable this in about:config
privacy.firstparty.isolate set to true

Easy to disable FPI with a click, but it won't enable FPI without restarting the extension.

Very nice add-on, works like expected, only Apple iCloud in the browser doesn't load, have to deactivate the add-on it prior, after loading one of the "apps" I can activate it again. But I rarely need this, only sometimes when I'm at work. Indeed a whitelist would be nice, but not possible like you statet below.

Does one thing. Does it well.

Simple and clean add-on, does what it needs to do with just a click (activate or de-activate).

Complete perfectly my other privacy add-ons and it has NO conflict with them (u-block origin, editThisCookie, bloody viking!, Grammarly, User Agent switcher, and few others).

Strongly recommended!