what appeared to be a loose firewall. stopped engine and found it to be a flexible motor mount. it would flex a quarter inch up and down.
Are you sure the motor mount is flexing? Is this something you notice with the cowling on or off? The reason I ask is because if you push your engine's shaft up or down with the cowling on it may make it look like the firewall or engine mount are loose when in fact it is the cowling itself that moves as you put pressure on it with your other hand.