coolant leakage from firewall, but not interior of car..

If its coming from the firewall it could be the area where the heater hose connects to the heater core. You need to have a shop check this because the heater core replacement is a huge job and it might just need new heater hoses https://repairpal.com/directory?address=94803&car_bran... and you can get estimates here https://repairpal.com/estimator/bmw/740i