Jamieson, Fausset and Brown on Isaiah 9:18-21

18-21. Third strophe.

burneth--maketh consumption, not only spreading rapidly, but also consuming like fire: sin is its own punishment.

briers ... thorns--emblem of the wicked; especially those of low rank (Is 27:4; 2Sa 23:6).

forest--from the humble shrubbery the flame spreads to the vast forest; it reaches the high, as well as the low.

mount up like ... smoke--rather. "They (the thickets of the forest) shall lift themselves proudly aloft [the Hebrew is from a Syriac root, a cock, expressing stateliness of motion, from his strutting gait, Horsley], in (in passing into) volumes of ascending smoke" [Maurer].