SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_promotions AS promotions ON storefronts.storefront_id = promotions.storefront_id 
WHERE 
  1 = 1 
  AND (
    promotions.promotion_id IN ('180')
  ) 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00032

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "storefronts",
          "access_type": "system",
          "possible_keys": ["PRIMARY"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "promotions",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "idx_storefront_id"],
          "key": "PRIMARY",
          "key_length": "8",
          "used_key_parts": ["storefront_id", "promotion_id"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100,
          "using_index": true
        }
      }
    ]
  }
}