This has to be one block lower or it winds up in the sky.

src/main/java/com/gmail/nossr50/util/ParticleEffectUtils.java

@@ -53,6 +53,7 @@ public final class ParticleEffectUtils {
 
     private static void fireworkParticleShower(Player player, Color color) {
         Location location = player.getLocation();
+        location.setY(location.getY() - 1.0);
         location.setPitch(-90);
 
         Firework firework = (Firework) player.getWorld().spawnEntity(location, EntityType.FIREWORK);