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Rated 5 out of 5 stars
I think this addon is made to resemble the behavior of Google Chrome. (When private browsing is toggled, another window comes out with all external access to local profile is blocked)
When the private browsing mode is "ON", you can't change the settings, you can only do this when the mode is "OFF"
Note: I am not the developer, I just share my user experience of this addon. If you have further questions, you better contact the developer.