Short Tactic Overview:

1. Spread out!
2. Heal player with Incinerate Flesh until debuff gone (60k in 25man). If this is not done, the player will explode and aoe damage all targets in a very large area.
3. Move to outer walls and move along them until debuff is gone if you get Legion Flames
4. Dispell "Fireball" debuff on tank
5. Ranged DPS has to kill the adds as fast as possible

Healing at Lord Jaraxxus as Priest

The most important thing for you in this fight is to dispell the debuff on the tank and heal the player with the Incinerate Flesh debuff. There is not to much damage on the raid as long the healers remove this debuff from the player.
It is a very big help to add Incinerate Flesh to grid, so you can see fast which player got this debuff.