“Make sure you stay calm.” The Hebrew is more literally, “Guard yourself; stay calm.”
“two stubs of smoldering firewood.” The Hebrew word translated “firewood” is ud (#0181 אוּד), and it can refer to a stick that was used to poke and stir the fire, which often caught fire itself, or to a log or piece of wood in the fire. It is hard to tell in this case which of the two meanings applies, but the likely meaning is simply a piece of firewood. The English word “firebrand,” which is used in many English versions, is unclear because “firebrand” is not used of a stick one stirs the fire with and although “firebrand” can refer to a piece of burning wood, that is a very uncommon use of the word. Versions that read “firewood” include the BBE, CJB, NIV, and NJB.
In Isaiah 7:4, the prophet Isaiah is telling King Ahaz that the kings of Syria and Israel are just smoldering pieces of wood. Their fire has gone out and soon they will be gone too; Rezin was killed by the Assyrian King Tiglath-Pileser III, and Pekah was assassinated by his successor, Hoshea (2 Kings 15:27-30).
“the son of Remaliah.” This refers to King Pekah, who was the son of Remaliah (2 Kings 15:25, 27). Pekah became king of Israel by killing King Azariah (2 Kings 15:25), and he himself was killed by Hoshea who became king in his place (2 Kings 15:30). Hoshea was the last king of Israel before it was conquered by Assyria.