what would cause an engine to leak coolant on the ...

gravity

if a hose is leaking...get a new hose, or get some hose sealant and fix that one

I agree with - and would have loved to pen the original response - Bubba. "Gravity - not just a good idea, it's the
law" - Fig Newton (Sir Isaac's second, and little known, cousin)

Could be a freeze plug also.  Or a leak at the rear of the intake manifold or head gasket.

good call bubba...gravity can cause a whole heap of problems in the modern automobile...

Is the radiator low at all or is it just the overflow tank?

Either way...
Check the overflow tank for cracks... Check it's hoses.  If the leak isn't apparent... well then....

Step one:take car to mechanic
Step two: ask to have coolant system pressure tested for leaks
Step three: Fix the leak

you need a tranny

From the firewall meaning on the back side of the engine compartment, where there are 2 lines coming from?

This is where the heater core is located, those fittings/hoses  leak ALOT.

If its actually on the firewall side of the motor, theres a good chance the Intake Manifold Gasket is leaking. This is extremely common on the V6 GM made engines, 3.1, 3.4, 3.8. The malibu is usually equip. with the 3.1 V6.  Right now there is a class action lawsuit against General Motor Company, having to do with the massive amount of customer forking out thousands of dollars to have this issue fixed.

Other wise, there are tons of coolant lines on the motor to cool everything off; one leak and you could see it leaking on the ground.