Popups can't be screenshotted

Menus and the app menu are separate OS-level windows, so they appear in neither a Marionette chrome screenshot nor a drawWindow rasterisation of the browser window. Capturing the whole screen instead is worse: it depends on window stacking and picks up whatever else is on your desktop. Every view shot captures is therefore an in-document surface.

You can still look at popups in watch, and inspect them with the Browser Toolbox. These separate popup windows cannot be captured. The standard screenshot set does include the window-modal quit-confirmation dialog.