Rated 5 out of 5 stars

Feature request :
Use a different tab color for a Private Tabs, so we can tell wich tab is Private without activate that tab and see the private icon in the menu bar.

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

Private tabs should be underlined:
(this is used for compatibility purposes to support most themes and extensions)

And you can try to change tabs appearance using following styles: