1)
When will fire go in front of Him?
1.
Rashi: In the war of Gog and Magog - "v'Nishpateti Ito b'Dever uv'Dam v'Geshem Shotef v'Avnei Elgabish Esh v'Gafris" (Yechezkel 38:22).
2.
Malbim: This is a metaphor; it will be as if fire comes out of the cloud around him, to punish the Resha'im with His Hashgachah.
2)
Why will the fire be "Saviv Tzarav"?
1.
Radak: It is lest they be able to escape the fire.
