Rated 4 out of 5 stars

First thing's first: after a morning spent debugging the issue, I found this extension kills the site icons from all browser tabs except internal ones (like Options and Add-ons) on Firefox 50.

The basic functionality is nice. No need to tie a temporary session to a whole another window. It would be nice to be able to pin a private tab with a starting URL so that it would remain across sessions.

This review is for a previous version of the add-on (0.1.9.1). 

See https://github.com/Infocatcher/Private_Tab/issues/224
Fixed version already released, but not yet reviewed:
https://addons.mozilla.org/addon/private-tab/versions/0.2.0