brent
There is actually not much use anymore for enabling the old FPI (outside of TOR browser maybe) with all the current isolation / blocking features Firefox has, especially containers and Enhanced Tracking Protection. And AFAIK FPI isn't seeing much improvement anymore (if at all) and disables all or most of the current isolation / blocking features.
You should be better off just setting Enhanced Tracking Protection to "strict", properly setting up which data to clear when Firefox gets closed and using uBlock Origin.
It will almost give you the same results (IIRC FPI still isolates a bit more currently) while using current isolation / blocking features but with a lot less breakage.
And if you wan to up it a notch you can use Multi Account Containers together with the extension Temporary Containers (in auto mode). But with the ongoing improvements to Enhanced Tracking Protection most privacy enhancements can now be gained without using Multi Account Containers, making them pretty much only useful for their core use case (which is being able to log into the same site with different logins at the same time using the same browser with different containers).
Here's some good info about Total Cookie Protection (which is part of Enhanced Tracking Protection):
https://github.com/mozilla/multi-account-containers/issues/1974
More info can be found in the release and feature announcements by Mozilla.
Also there is work being done deprecating the old FPI:
https://bugzilla.mozilla.org/show_bug.cgi?id=1649876
So at one one point enabling it (if the setting is still there) will most likely just enable the current protection features you can already enable from the normal settings menu.
