It is well-known that the tefillin shel yad - that which is placed on the arm - represents subservience of one's strength and might to Hashem's service. This is why the bayis (box) goes directly on the biceps muscle. The tefillin shel rosh - that which is worn on the head - symbolizes the commitment and subservience of one's mind to God's Will.
Thus, tefillin are central insomuch as they physically designate us as servants of God in terms of our physical and intellectual capacities. This theme is identical with that of the mitzvos of Passover and the laws of the firstborn, as these commandments display our commitment and dedication to God's service after being released from bondage to Pharaoh. We were liberated from Egypt and acquired by God, raised and sanctified for direct service to Hashem. Tefillin are a brand of servitude to God, and they thus eternally complement the attachment and commitment to Hashem as expressed by Passover and the sanctification of the firstborn.