Rated 5 out of 5 stars

works as expected, very nicely: as an example, it allows to keep a tab logged into Gmail, while searching privately Google on another tab.Nice to have: to open all new tabs in private mode by default (I mean: all new blank/empty tabs)

This review is for a previous version of the add-on (0.1.0b2.1-signed). 

> Nice to have: to open all new tabs in private mode by default
What you mean?
All new blank/empty tabs or every new tab (empty or not)?
In second case you can try built-in permanent private browsing mode:
Options – Privacy – Firefox will: Use custom settings for history – Always use private browsing mode

[Added at 2013-04-02]
I will try to implement this feature, see https://github.com/Infocatcher/Private_Tab/issues/10 for progress

[Added at 2013-04-09]
See extensions.privateTab.makeNewEmptyTabsPrivate and extensions.privateTab.makeNewEmptyWindowsPrivate preferences in new version:
https://addons.mozilla.org/addon/private-tab/versions/0.1.0