SELECT 
  company_id, 
  storefront 
FROM 
  cscart_companies 
WHERE 
  FIND_IN_SET('US', countries_list) 
LIMIT 
  1

Query time 0.00009

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}