Rated 5 out of 5 stars

Does what it's supposed to do, cleanly and flawlessly.

Quick question: would there be any possibility of adding an option to make the color of a private tab -- the tab itself, up there in the tab toolbar -- be different from the rest of the tabs? It would be very useful to be able to just glance at the tab toolbar and see if there were any private tabs open.

Edit: thanks for the pointer to the styles! They work like a charm in userChrome.css.

This review is for a previous version of the add-on (0.1.7.3.1-signed). 

See some example styles for userChrome.css/Stylish here:
https://github.com/Infocatcher/Private_Tab#styles
(not so good in Firefox 29+ with Australis, but not bad as a start point...)