This doesn't appear to present a security issue to me or else it is hopefully obvious I would have used a private channel for disclosure.
I recently right clicked on a link to PA and selected Open Link in New Window which the browser promptly did...unfortunately It also opened a second tab to the warning page about opening a Gist while logged into PA. Since I had looked at a link earlier to a PA-Gist on SO from Giles, I wondered if I could duplicate the behavior and unfortunately I could.
When I right click on PA and select Open Link in New Private Window followed by a second time in just a new window, I get a second tab.
When I right click on PA and select Open Link in New Private Window N time followed by a final time in just a new window, I get N additional tabs.
I started to look at my cookies and such during the various parts of this, but got distracted, so I figured I'd at least post it here quick. If you have any trouble duplicating then of course I'll provide any additional information you require.
Windows x64 7 Home Premium FF Aurora 21.0a2 (2013-03-27)