Fix collisions passing firestacks between mobs (#9324)

Content.Server/Atmos/Components/FlammableComponent.cs

@@ -1,4 +1,5 @@
 using Content.Shared.Damage;
+using Robust.Shared.Physics.Collision.Shapes;
 
 namespace Content.Server.Atmos.Components
 {
@@ -28,5 +29,11 @@ namespace Content.Server.Atmos.Components
         [DataField("damage", required: true)]
         [ViewVariables(VVAccess.ReadWrite)]
         public DamageSpecifier Damage = new(); // Empty by default, we don't want any funny NREs.
+
+        /// <summary>
+        ///     Used for the fixture created to handle passing firestacks when two flammable objects collide.
+        /// </summary>
+        [DataField("flammableCollisionShape")]
+        public IPhysShape FlammableCollisionShape = new PhysShapeCircle() { Radius = 0.35f };
     }
 }