second private window quit with Cmd-Shift-W will quit firefox completly

[childnode]

steps to reproduce:

1. open Firefox
2. open a second inkognito window (cmd-shift-P)
3. press cmd-shift-w double
4. both windows and the instance is closed

expected: only the private mode window is closed as it does on the red window close button, and the menu entry close window

version: Firefox 39.0 and warn-before-quit 0.7