Just what I needed.

It works, just enable it in Private Browsing.

Not working on a new Laptop. I synced firefox on a new laptop and it's not working there somehow
Edint: My bad, you have to allow it to work on private mode, duh

I thought it wasn't working because other methods remove the option for a private window - it isn't even there. This one closes the window as it opens. So it sort of taunts the user... which is amazing.

Works without trouble. Simple but effective, just how it should be.

it works

Does what it says. It does not allow for private windows to launch in Firefox.