Sha'ar HaBechorot - Gate of the firstborn

Sacrifices are divided into tow types: those of greater and those of lesser sanctity. Those of greater sanctity had to be slaughtered north of the Altar. Those of lesser sanctity could be slaughtered anywhere in the Azarah, even in the southern area.

The firstborn offering was once of the more common sacrifices of lesser sanctity and was therefore brought in through a southern gate to indicate its relative unimportance.

Some say that when Abraham brought Isaac, his firstborn, to be sacrificed upon the mount he passed this way.