Above this floor was God seated on his throne. Yet John does not mention God or this throne simply because God is too awesome for human eyes to settle on. As we look beside the sun (as opposed to looking directly at the sun), so John looked beside/under God, and that is what he describes (see also Revelation 4:6; Ezekiel 1:22). So we are to understand that in this vision John was in the very presence of God.
2 And I saw what appeared to be a sea of glass mingled with fire and also those who had conquered the beast and its image and the number of its name, standing beside the sea of glass with harps of God in their hands.