How could fire be contained inside 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.
Seforno (on Pasuk 23): Which due to the same swiftness of the hail descended down to earth.
What does "Mislakachas b'Soch ha'Barad" mean?
Hadar Zekenim, from Pesikta: It means that they formed a sort of mixture.
Targum Yonason: It means that the fire jumped about inside the hail.
Chizkuni: After the hail struck someone, the fire took him (Lokcho) and burned him.
Ba'al ha'Turim: Sparks and light came out. 1
Ba'al ha'Turim: Like "ve''Eish Mislakachas ve'Nogah Lo Saviv"