Rated 5 out of 5 stars

Awesome! Was expecting this for a long time.
Would you consider adding localization?

This review is for a previous version of the add-on (0.1.0.1-signed). 

Some localizations already added in development version: https://github.com/Infocatcher/Private_Tab/tree/master/locale

And you can translate it yourself:

1) locale/en-US/pt.properties
https://github.com/Infocatcher/Private_Tab/blob/master/locale/en-US/pt.properties
2) extension description in install.rdf
https://github.com/Infocatcher/Private_Tab/blob/master/install.rdf
(create new <em:localized>...</em:localized> block)

And then "propose" changes like described here:
https://github.com/Infocatcher/Private_Tab/issues/11#issuecomment-15774546