gistfile1.java

 if (UserAgent.isFirefox()) {
      popup =
          PopupFactory.createPopup(this.getElement(), new RelativePopupPositioner() {
            
            @Override
            public void setPopupPositionAndMakeVisible(Element relative, final Element p) {
              ScheduleCommand.addCommand(new Scheduler.Task() {
                @Override
                public void execute() {
                  p.getStyle().setLeft((RootPanel.get().getOffsetWidth() - p.getOffsetWidth()) / 2, Unit.PX);
                  int top = (RootPanel.get().getOffsetHeight() - p.getOffsetHeight()) / 4;
                  p.getStyle().setTop(Math.max(top, 280), Unit.PX);
                  p.getStyle().setVisibility(Visibility.VISIBLE);
                }
              });
            }
          }, chrome,
              true);
    } else {
      popup = PopupFactory.createPopup(null, new CenterPopupPositioner(), chrome, true);
    }