SELECT 
  storefronts.* 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_companies AS companies ON storefronts.storefront_id = companies.storefront_id 
WHERE 
  1 = 1 
  AND (
    companies.company_id IN ('1')
  ) 
GROUP BY 
  storefronts.storefront_id 
ORDER BY 
  storefronts.is_default desc

Query time 0.00013

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE storefronts system PRIMARY 1
1 SIMPLE companies system PRIMARY,idx_storefront_id 1

Result

storefront_id url redirect_customer is_default status access_key name theme_name is_accessible_for_authorized_customers_only
1 cs-cart.pixy.pro N Y N CS-Cart bright_theme N