How have you disabled Private Browsing?

There's an add-on I downloaded that disables deleting your history and opening the private browser

...which will be easily circumventable by pressing the shift-key while firefox is launching and going into safemode. there will be no reliable way to prevent users from clearing their history in firefox.

As philipp mentioned, the Disable Private Browsing extension is easily overriden by opening Firefox in the SafeMode.

I helped RichieB the developer of Disable Private Browsing do beta testing was it was ready for users. The Add-on approval team didn't like one aspect of that extension (hiding the extension from the Addons window so users wouldn't even know that it was installed - only showed when in SafeMode) and RichieB had to remove that feature before they would allow DPB on the Addons website. They made a new "rule" about not hiding an extension in the UI after seeing that extension.

There is a Bug report (#1269296) that was filed in May 2016 that requested a feature to remove the Forget button. There was mention of adding a WebExtenbsions API; other comments mentions it being made part of an "enterprise policy" which isn't in the current initiative for Web Extensions. And a few developers mentioned that they "didn't see the point" in adding that type of feature to Firefox. Hasn't been any progress towards making a decision about that feature; it will probably get a WONTFIX category.

All I can suggest is to move it into Additional Tools & Features via Customize or hide it via userChrome.css rules. http://kb.mozillazine.org/UserChrome.css Sorry, I don't know the code that is needed to hide that button.