Is it possible to reposition the group window pop up?

[winteriscariot]

I'd like to anchor these pop ups in the middle of the firefox icon, is that possible via css?

[LBCrion]

Sure. I added properties -GtkWidget-widget-anchor and -GtkWidget-window-anchor to the latest git version. These allow you to explicitly specify how a popup window will be positioned vs the parent widget. The possible values are northwest, north, northeast, east, center, west, southwest, south, southeast, static and default.
The properties specify a specific edge or a corner of the widget that will be aligned with a specific edge or corner or a window. I.e. to have a center alignment of a window above widget you want to specify -GtkWidget-widget-anchor: north and -GtkWidget-window-anchor: south. So the southern (bottom) edge of the window is attached to the northern (top) edge of a widget.