Rated 4 out of 5 stars

Thank you for this addon. I have a suggestion.
Would you consider adding a "Search Google in New Private Tab..." menu item for highlighted text? Sometimes it's nice to search for selected words and phrases online without it appearing in my Google history. :)

This review is for a previous version of the add-on (0.2.2). 

You can use special bookmark with keyword.
Location: private:https://www.google.com/search?q=%S
Keyword: pg (Private Google)

Then type in URL bar:
pg some request

See http://kb.mozillazine.org/Using_keyword_searches