It is possible disable Private Browsing mode via a policies.json file or via GPO, so maybe that is active in your case. Some security software might be adding such rules to Firefox. You can check the about:policies#active page to see if the are policies active.

• https://github.com/mozilla/policy-templates/blob/master/README.md
• https://support.mozilla.org/en-US/kb/customizing-firefox-using-group-policy
• https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson

{
  "policies": {
    "DisablePrivateBrowsing": true
  }
}

"about:policies#active"

Leads to a invalid URL page

Can you open the internal page

about:about

and click the link the about:policies on that page?

Note: about:about is the entire address, there's no http:// or .com at the end.

My about:about result is attached

jiten87 said

My about:about result is attached

Hi jiten87, is that in Firefox 64, or Firefox 60 ESR, or a different version?

see attached image

The about:policies page has only recently been added to Firefox and isn't present in Firefox 61, but your screenshot shows that at least updates are disabled in your Firefox version likely via GPO.

You can inspect the Mozilla and Firefox keys in this location to see whether GPO policies are active:

• HKEY_LOCAL_MACHINE\SOFTWARE\Policies\

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Firefox

This is what im seeing for that registry folder

That screenshot confirms that Private Browsing mode is disabled by GPO.

Is this your own computer or is this device setup by an IT department?

This is not a personal computer

Under the circumstances, you probably should use a different device for your truly private browsing.

Thanks everyone for narrowing this down.