Jamieson, Fausset and Brown on Judges 15:4-5

4-5. went and caught three hundred foxes--rather, "jackals"; an animal between a wolf and a fox, which, unlike our fox, a solitary creature, prowls in large packs or herds and abounds in the mountains of Palestine. The collection of so great a number would require both time and assistance.

took firebrands--torches or matches which would burn slowly, retaining the fire, and blaze fiercely when blown by the wind. He put two jackals together, tail by tail, and fastened tightly a fire match between them. At nightfall he lighted the firebrand and sent each pair successively down from the hills, into the "Shefala," or plain of Philistia, lying on the borders of Dan and Judah, a rich and extensive corn district. The pain caused by the fire would make the animals toss about to a wide extent, kindling one great conflagration. But no one could render assistance to his neighbor: the devastation was so general, the panic would be so great.