Charles Ellicott Commentary Psalms 99:5

"Exalt ye Jehovah our God, And worship at his footstool: Holy is he." — Psalms 99:5 (ASV)

Worship at His footstool. — Prostrate yourselves at His footstool. The earth is called the “footstool” of God (Isaiah 66:1); in other places the expression is used of the sanctuary (Psalms 132:7; Lamentations 2:1). In 1 Chronicles 28:2 it seems to refer to the ark. No doubt here, after mentioning the throne above the cherubim, we must think of the ground on which the ark stood, or of the ark itself.