Ancalagon
Dusty Dragon
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.
What about thorn whip? Would you consider the magically created whip to constitute a physical connection between the attacker and the target?
If the druid is within 5 feet of the target, they would take fireshield damage, because the whip emanates from the druid. The druid is where the spell lashes out.