Rated 5 out of 5 stars

Excellent add-on. Nice and simple to use. One minor thing. You could add an option to disable the underline and the mask thingy. And make an option replacing the favicon with a mask, just like Opera 12.

This really needs to be a built-in feature in Firefox. Too lazy to open a new window :P

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

> You could add an option to disable the underline and the mask thingy.
Currently no option to remove the mask, sorry.
See https://github.com/Infocatcher/Private_Tab/issues/40
And example style: http://forums.mozillazine.org/viewtopic.php?p=12830693#p12830693
Or for classic menu:
#main-window[privatebrowsingmode=temporary] #toolbar-menubar {
  background-image: none !important;
}

> And make an option replacing the favicon with a mask, just like Opera 12.
See https://github.com/Infocatcher/Private_Tab#styles
To remove underline: https://github.com/Infocatcher/UserStyles/blob/5295618b1689c06c022b4646d3f0810e1ee07368/Private_Tab_background/privateTabBackground-gradient.css#L88