SELECT 
  COUNT(
    DISTINCT storefronts.storefront_id
  ) 
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 ('1')
  )

Query time 0.00248

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.008506245,
    "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": 1,
          "cost": 0.006792605,
          "filtered": 100,
          "attached_condition": "currencies.currency_id = '1'",
          "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": ["lvshop.currencies.storefront_id"],
          "loops": 1,
          "rows": 1,
          "cost": 0.00171364,
          "filtered": 100,
          "attached_condition": "storefronts.storefront_id = currencies.storefront_id"
        }
      }
    ]
  }
}