SPECIAL TOPIC: "MY FOOTSTOOL" (BDB 919)

This imagery has three orientations.

Earth is YHWH's footstool (i.e. He is the Creator and King, cf. Isa. 66:1).
The Ark of the Covenant is YHWH's footstool (the temple is where His Presence resides, cf. 1 Chr. 28:2; Ps. 99:5; 132:7; Lam. 2:1).
SPECIAL TOPIC: THE ARK OF THE COVENANT
YHWH's enemies and the enemies of His people will be His footstool (i.e. imagery of victory, cf. Ps. 110:1; Matt. 22:44; Mark 12:36; Luke 20:43; Acts 2:35; Heb. 1:13; 10:13).