EZEKIEL 1:22
And the likeness of the firmament on the heads of the living creature was as the color of the terrible crystal, stretched forth over their heads above. [AKJV]
The Firmament here means "Heaven" "Sky" or "Ceiling" over the Rabim. But that Firmament over these Living Creatures is different from this firmament (heaven/sky) over us on this earth.
This our own heaven (sky or firmament), contains the stars, sun, moon, physical particles, space and planets. But this our own heaven or firmament is not sustainable for these Living Creatures to operate under because they operate under the Heaven or Firmament that can abide both the Throne of God and these Living Creatures in their full manifestations in glory.
In other words, this our own heaven or firmament that scientists, witches, astrologers, crafts, sun, stars, etc, play in, do not have what it takes to contain the Full Manifestation of God, His Throne, and these Living Creatures. Unless God comes under this our own sky in His reduced form as Man, as Son of man, as an Angel in reduced form, or He appears in a Fire, Clouds, Wind or Visions/Dream; or He descends as the Word of God or Spirit of God, before this heaven or sky may abide His Presence. This implying that God humbles Himself each time God comes down under this heaven or to earth. This act of God humbling Himself to come down to us was not just that which Jesus Christ did as recorded in "Philippians 2:6-8;" because God the Most High always Humbles or Reduces Himself in order to fit into the size and capacity of safely descending and interacting with people and things under this heaven and on earth. As it is written,
[Psalms 113:4-6] — "The LORD is high above all nations, and His glory above the heavens. Who is like to the LORD our God, who dwells on high, He humbles Himself to behold the things that are in heaven, and in the earth!"
Therefore, the Day God will show Himself in Full Manifestation on His Throne, without Humbling/Reducing Himself into this Realms, this Heaven and Earth will be destroyed that day. As it is written,
[Revelation 20:11] — "Then I saw a great white throne and the One seated on it. Earth and heaven fled from His Presence, and no place was found for them." (BSB)
[Isaiah 34:4] — "All the stars of heaven will be dissolved. The skies will be rolled up like a scroll, and all their stars will fall like withered leaves from the vine, like foliage from the fig tree." (BSB)
Hence, the Living Creatures operate under different Firmament or Heaven that could abide the Throne of God and He who sits on It. That is the Firmament that can abide the Thunderings, Lightnings, Fire/Flames, loud Voices and Visions proceeding from the Throne of God, without that Firmament quaking or shaking into collapse. Remember at the beginning of this revelation to Ezekiel, the Heavens were opened and Ezekiel saw visions of God. As written,
[Ezekiel 1:1] - "In the thirtieth year, in the fourth month, on the fifth day, while I was among the exiles by the River Kebar, the heavens opened and I saw visions of God."
Here's the description of the Heaven, Firmament or Ceiling, stretched over the Heads of the Living Creatures according to the top Bible verse above.
[Ezekiel 1:22] — "And the likeness of the firmament on the heads of the living creature was as the color of the terrible crystal, stretched forth over their heads above."
That Firmament (Heaven) is like the "Color of the Terrible Crystal." Two questions: If it's a Crystal, why is it having Color when a Crystal is basically Colorless? And is there any Crystal that is termed "Terrible Crystal" or "Terrible?"
This Firmament over the Living Creatures is like a Crystal (Glass or Ice) that reflects the Glory and Countenance of God. So both the color and the terribleness (dreadfulness) of the Crystal (Glass) like Firmament over the Rabim are reflections, mirroring, declaring the Glory and Countenance of God. This is where you understand better, what Paul said:
"We see through a Glass darkly…" - (1 Corinthians 13:12)
Or Paul said:
"But we all, with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory, even as by the Spirit of the LORD." - (2 Corinthians 3:18)
But much more to understand even that Firmament over the Heads of the Living Creatures is a Firmament, Crystal or Glass that declares the Glory and Countenance of God. As it is written,
[Psalms 19:1-2] — “The heavens declare the glory of God; and the firmament shows His Handiwork. Day to day utters speech, and night to night shows knowledge.”