‘And when forty years were fulfilled, an angel appeared to him in the wilderness of Mount Sinai, in a flame of fire in a bush. And when Moses saw it, he wondered at the sight, and as he drew near to gaze at it, there came a voice of the Lord, “I am the God of your fathers, the God of Abraham, and of Isaac, and of Jacob.” And Moses trembled, and did not dare to look.'
And God had appeared in fire, and had spoken to him declaring that He was the God of his fathers, the God Who had made His promises to Abraham (Acts 7:5). His promises of a deliverer were now about to be fulfilled (Acts 7:7). And Moses had wondered at the sight and had trembled, not daring to look on God.
(In the same way God had revealed Himself in fire at Pentecost. The God of Fire was again offering deliverance if only they would respond. Perhaps Stephen also saw a connection between the forty years of Moses and the forty days of Jesus resurrection appearances - Acts 1:3).