And I saw, as it were,
John continues on looking (Revelation 4:1). He is aware of what is going on. Seeing, observing, being alert is not optional for a believer. It is required:
a SEA OF GLASS mixed with FIRE, and
The SEA of GLASS is symbolic of the victorious Christ upon whom we can STAND. The fire denotes pending judgment (Hebrews 12:18-29).
Glass is formed by heating up silicates, and this SEA of GLASS apparently is getting hotter and hotter. Compare this SEA of GLASS with the one around the throne (Revelation 4:6). They probably are the same but this one reflects the 7 pending bowls of judgment. Hence mixed with FIRE.
those who had come off VICTORIOUS from the BEAST and from his IMAGE and from the NUMBER of his name,
Each of the seven churches had blessings for their victorious overcomers (Revelation 2:7, 2:11, 2:17, 2:26, 3:5, 3:12, and 3:21). However, these VICTORIOUS ones are not those from the seven churches. Rather they are the ones who persevered through the beast's deception (Revelation 13:10 and 14:12). They did NOT take the NUMBER (Revelation 13:18).
STANDING on the sea of glass, holding HARPS of God.
The beginning of the seventh trumpet finds humble worship (Revelation 11:16) face down and totally subservient. These saints are STANDING tall as witnesses of God's final judgments. It is for them, in particular, that these judgments are done. They observe with unblinking awe that they made the right choice and that God is this powerful. They probably knew it all along but didn't realize it until this moment.
Look how Daniel sees this moment (Daniel 12:1-3-4).
The HARP appears to be "the God instrument" (Psalm 33:1-2-3 and Revelation 5:8). The devil knows this and apparently tries to reproduce the same sound/music in Babylon (Revelation 18:22). However, I have a feeling the beast/devil has no idea how to play the HARPs like these STANDING here do!