출애굽기 3:2 해설: Rashi, Chizkuni, Da'at Zekenim & Ramban

בלבת אש means IN A FLAME OF FIRE, i. e. in the very heart of (לב) the fire. Similarly is לב used of inanimate objects in: (Deuteronomy 4:11) “in the heart of (לב) heaven”; (II Samuel 18:14) “in the midst of (לב) the terebinth”. Do not be puzzled by the ת, although the ordinary word for heart, לב. does not require the suffix ת in the construct form, because we have another example of this form, viz., (Ezekiel 16:30) “how weak is thy heart (לבתך)”.‎

בלבת אש, “in the heart of the fire;” G-d wanted Moses to get used to such a phenomenon so that when the time came for the revelation at Mount Sinai, he would not become frightened by either it or the lightning.

”out of the burning bush.” The reason that G–d chose this bush to reveal Himself in was that one could not construct a deity or symbol of a deity out of the bush.