Extremely happy with this addon, and have used it successfully for many years in Firefox 😊 Apparently though, in modern Firefox, privacy.firstparty.isolate = false is the default because it is all handled automatically by other processes. However, in Mullvad browser, privacy.firstparty.isolate = true - is the default because they have a stricter configuration. So there is a distinction between these two Firefox-based browsers and how they deal with the privacy.firstparty.isolate issue.

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.