Seems to be working Rated 4 out of 5 stars

But it must be used with "public fox" extension to prevent uninstalling it. Public fox asks for a password to enter settings, to delete history and to enter the extension panel.
However, a smart user can stil run firefox with creating a new profile (extensions are per profile, so there will be none in a new profile) and keep surfing in private mode.
Therefore the best method to disable private bowsing is to phisycally remove the file nsPrivateBrowsingService.js from Program Files / Firefox folder and taking away the administrative rights, so the user will not be able to reinstall firefox.

This review is for a previous version of the add-on (1.7.1-signed).