How could fire be contained amid the hail?
Rashi: This was a miracle within a miracle - two opposing forces made peace in order to fulfill the will of their Creator.
Rashbam: Refer to 9:14:1:3.
Seforno: It wasn't. What happened was that the power of the swiftly moving hailstones heated up the air, turning it into fire 1 and created thunder.
Which due to the same swiftness of the hail descended down to earth (Seforno, on Pasuk 23).