SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_currencies AS currencies ON storefronts.storefront_id = currencies.storefront_id 
WHERE 
  1 = 1 
  AND (
    currencies.currency_id IN ('4')
  ) 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00024

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.012670745,
    "filesort": {
      "sort_key": "storefronts.is_default desc",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "currencies",
              "access_type": "index",
              "possible_keys": ["PRIMARY", "idx_storefront_id"],
              "key": "idx_storefront_id",
              "key_length": "4",
              "used_key_parts": ["storefront_id"],
              "loops": 1,
              "rows": 5,
              "cost": 0.007379345,
              "filtered": 100,
              "attached_condition": "currencies.currency_id = '4'",
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "storefronts",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "4",
              "used_key_parts": ["storefront_id"],
              "ref": ["u428615623_ecartifygonje.currencies.storefront_id"],
              "loops": 5,
              "rows": 1,
              "cost": 0.0052914,
              "filtered": 100,
              "attached_condition": "storefronts.storefront_id = currencies.storefront_id"
            }
          }
        ]
      }
    }
  }
}