Rated 5 out of 5 stars

Great addon !

Question/suggestion - can we in future have ability to open private tabs in a "backgound" private tab via the contextual menu? ie have the option "Open Link in Background Private Tab"

This review is for a previous version of the add-on (0.1.5). 

By default Private Tab uses built-in browser.tabs.loadInBackground preference from about:config page.
Or you can change extensions.privateTab.loadInBackground to 1, see https://github.com/Infocatcher/Private_Tab/blob/master/defaults/preferences/prefs.js#files

Also you can middle-click on "Open Link in New Private Tab" item to invert background/foreground behavior.