And I saw as it were a sea of glass mingled with fire: and them that had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, stand on the sea of glass, having the harps of God.(Revelation 15:2) KJV
This is a very similar idea to what we saw in Revelation 21. The multitudes of the saved are standing on something described as a sea of glass which, contextually, we now know is the foundation of heaven which is the firmament.
- sea of glass