In Firefox (56.0 (64-bit) on macOS 10.12.6), when a gmail window is popped out (e.g. for composing an email) it ends up taking up only the top-left quadrant of the window. The rest of the window is not part of the document (it's not clickable, ie. right-clicking produces no action).
Weirdly, if I right-click inside the gmail window, the menu appears in the place I would expect that part of the window to be (i.e. if I right-clicked at
(0.9,0.9) in the gmail "view", then the menu would appear in
(0.9,0.9) of the window).