Rashi: Your astrologers will burn like straw. "Ein Gacheles Lachmam" - there is no remnant for them, i.e. coals to get warm via their fire.

Radak: Fire burns it quickly. They cannot save themselves from the flame that burned them, i.e. the affliction. How can they save you?! They are unlike wood that burns; there is benefit from its coal. Rather, they are like straw, which has no benefit - it has no coal, and its flame does not endure. So these astrologers - when the affliction came, their Chachmah did not help themselves and not others.

Malbim: Straw is inferior to wood in two ways. (a) Wood burns only due to a burning fire near it, but from a flame far away. Straw burns even from a far flame - "Esh Saraftam... mi'Yad Lehavah." (b) There is benefit from wood that burns - light, and one can be warmed from its coals. Straw does not leave light nor coals.

Radak: It is Makor (like an infinitive), to get warmed from it. Its grammatical form is like "Lishkav Es Bas Yakov" (Bereishis 34:7). There is a Patach under the Lamed due to the Ches after it.

Radak, Malbim: There is no light; "Ein" applies also to this. Ohr is the flame that burned them. They received no benefit from it, to sit opposite it to get warm. So when the affliction came to them, they found no place of benefit.