由 Infocatcher （開發者） 發表於 Sept. 10, 2015 · 永久網址

> 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.