Make sure you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/Private+Browsing

• Options/Preferences -> Privacy & Security -> Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"

Firefox 42+ has a Tracking Protection feature that is enabled by default in Private Browsing mode. Firefox can show a shield icon at the left end of the location/address bar that some content is blocked by Tracking Protection, click the shield icon for more information.

You can disable this feature in "Options/Preferences -> Privacy" or via the about:privatebrowsing page that get when you open a New Private Window.

• Options/Preferences -> Privacy: "Use Tracking Protection in Private Windows"

• https://support.mozilla.org/en-US/kb/tracking-protection-firefox
• https://support.mozilla.org/en-US/kb/control-center-site-privacy-and-security-firefox

Cor-el - I'm fairly sure that this user wants to disable Private Browsing so that it is not available to other users(e.g. children). My answer is based on that presumption. Most things are easily reversed. The other real problem is that quite young users, for example, can easily instal and later remove an entirely different browser. Also with non-private browsing it is not too difficult to remove most traces. There used to be regular answers with protection procedures but I can't remember the details and there always seemed to be ways around them. The computer could be turned off behind a password and used only when supervision is available.

There is no way to disable PB mode reliably if that is what you are after. Previously there were Legacy extensions that could do this, but those can easily be bypassed. The same with hiding nu items or disabling Options/Preferences. You can disable permanent Private Browsing mode by locking the related pref (browser.privatebrowsing.autostart) to false via mozilla.cfg, but that still won't prevent opening a New Private Window.

For instance, if you open the about:privatebrowsing page then you get a button to open a PB mode window.

If you want to monitor what others (children) are doing on a computer then you need to use monitoring or parental software at the OS level that can't be bypassed. Browsers aren't designed and suitable to do this reliably.

Still assuming I'm on the right track, cor-el has come up with the right lead. Other users may come up with recommendations for "parental controls at operating system level"(Mac in your case) . Always ensuring that another operating system can't be accessed. Not aimed at you specifically but users in general should expect that 12 year-olds know at least as much as they do.

Response?