Rated 4 out of 5 stars

Very nice addon.
There's only one thing that hit me, I don't know if it's a bug or not.
When the current tab is private and that I open a new tab from bookmarks with a middle click it opens as private too.

This review is for a previous version of the add-on (0.1.8). 

> When the current tab is private and that I open a new tab from bookmarks with a middle click it opens as private too.

This is known issue:
https://addons.mozilla.org/addon/private-tab/#developer-comments
=> https://github.com/Infocatcher/Private_Tab#known-issues
This is due to balance between stability (and compatibility with another extensions) and with usability.