Hriston
Dungeon Master of Middle-earth (He/him)
Okay, you can create exceptions to fire shield by adding the requirement that there must be a physical connection between the caster and the attacker for the spell to work. This would also except Mordenkainen's sword from triggering damage from fire shield.While I've pointed out why I disagree. There are exceptions to every rule, I think this is one of them.
In every case that I know of other than spiritual weapon the attacker or an object, which is typically a weapon but could be an implement or a hand, needs to physically touch the creature with fire shield.
Since the spiritual weapon is not physically attached to the caster I rule that they are unaffected. Feel free to rule differently.
What about thorn whip? Would you consider the magically created whip to constitute a physical connection between the attacker and the target?