From pur; to kindle, i.e. (passively) to be ignited, glow (literally), be refined (by implication), or (figuratively) to be inflamed (with anger, grief, lust) -- burn, fiery, be on fire, try.
KJV: Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat?
NASB: looking for and hastening the coming of the day of God, because of which the heavens will be destroyed by burning, and the elements will melt with intense heat!