Use the Disable Private Browsing add-on.

how can i use Disable Private Browsing, i downloaded it but where can i find it?

After install and restart Firefox the add-on start to work automatic, try open a private window.

There is an addon that achieves this

• https://addons.mozilla.org/en-US/firefox/addon/disable-private-browsing/

There are ways around that. However also note there are all sorts of other security measures needed to lock down computers and that is beyond the scope of this forum . Many schoolkids or office workers would be able to install an additional browser or a whole OS on a computer and if necessary run the OS or browser from a memory stick for instance.

(edit typos & expanded answer slightly )

Thank You all :) :) :) :)