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).
What is "Mislakachas [b'Soch ha'Barad"]?
Hadar Zekenim, from Pesikta: It is a mixture (like water and oil mixed, and fire exudes from them).
Targum Yonason: It is jumping.
Chizkuni: After the hail hit [someone], the fire was Lokcho (took him) and burned him.
Ba'al ha'Turim: Sparks and light came out like "v''Esh Mislakachas v'Nogah Lo Saviv."
Fire came between hailstones. (It seems that Rashbam (14) explains so.)