Why did God appear on the mountain? To give His law to the people! And why did He appear in a fire? Because what is from this world cannot escape fire. Because there is nothing in this world that can survive in the presence of God! Therefore, we either obey His word and may have fellowship with God, or we disobey, but then His fire consumes us.
His Word is eternal
Although many believe that the Father has changed since Jesus' death and resurrection. But if God had really changed, He can change anytime again, and then our faith would not be rock solid. But the good news is that God does not change, although for many it is bad news and judgment at the same time.