Fiery Soul Trap has no effect on the normal Fire Damage enchantment. Basically, what it does is that in addition to the Fire Damage enchantment you already have, Fiery Soul Trap does 10 points worth of Fire Damage (this isn't affected by enchanting skill, perks, or type of soul used, though it is affected by Augmented Flames, Fire Enchanter, and Intense Flames) on top of the normal Soul Trap effect (unlike the 10 points of Fire Damage, this can be improved by enchanting skill, perks, and type of soul used, just like the ordinary Soul Trap). More information can be found here.
Fiery Soul Trap has no effect on the normal Fire Damage enchantment. Basically, what it does is that in addition to the Fire Damage enchantment you already have, Fiery Soul Trap does 10 points worth of Fire Damage (this isn't affected by enchanting skill, perks, or type of soul used, though it is affected by Augmented Flames, Fire Enchanter, and Intense Flames) on top of the normal Soul Trap effect (unlike the 10 points of Fire Damage, this can be improved by enchanting skill, perks, and type of soul used, just like the ordinary Soul Trap). More information can be found here.