Isaiah delivers a message of assurance to king Ahaz. Ahaz is not to worry about the belligerent intentions of Syria and Israel (northern kingdom). The two kings are mentioned here, and then information about them continues in the next several verses. While Pekah is named specifically in verse 1 he is now referred to only as the son of Remaliah. Ludlow suggests that this is a sign of Isaiah’s disdain for the son or Remaliah, to whom he does not now refer directly but rather indirectly (Ludlow, p. 141).
The smoking firebrands are quite clearly the belligerent kings of Syria and Israel. It is possible that the “tails” refers to the recent defeat of the two kings and their retreat. Isaiah is emphasizing their defeat and attempting to quell Ahaz’s fears of their future intentions.