Rated 4 out of 5 stars

It would be useful to put an icon(Firefox default?) in the tab to indicate that it is in private mode, otherwise it is difficult to identify those specific privative tabs from other tabs when there are many tabs open.

This review is for a previous version of the add-on (0.1.6.1-signed). 

You can found some styles to change appearance of private tabs here:
https://github.com/Infocatcher/Private_Tab#styles