see http://kb.mozillazine.org/User:Dickvl/Private_Browsing_disable you will have to install the addon from the text posted, it is not at addons, but the instructions are clear.

This new extension disables Private Browsing, hides all the PB menu items and disables the keyboard controls, disables the clearing of browsing history, and the extension doesn't appear in the Add-ons except for the Firefox SafeMode.
https://addons.mozilla.org/en-US/firefox/addon/disable-private-browsing/

That does exactly the opposite of what I want. I want to PERMANENTLY ENABLE PRIVATE BROWSING and keep people from disabling it.

Here is a modified version of PB Disable that should do the opposite.
You can save the file as pbpermanent.xpi

data:application/octet-stream;base64,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==