Okay, I just had to work this out, because I'm like that. In 3E, the odds of a kobold with 1d8 hit points surviving a fireball cast by a 5th-level wizard depend on the wizard's Intelligence:
Wizard has Int 13 (minimum required to cast fireball): 1 in 37
Wizard has Int 16 (typical for a wizard built with the "elite array", with level 4 stat bump): 1 in 49
Wizard has Int 19 (typical for a point buy wizard, with level 4 stat bump): 1 in 59
Wizard has Int 25 (grey elf using point buy and fox's cunning): 1 in 141
So, it's actually not as unlikely as I would have expected--mostly because of the possibility of extra-tough kobolds that roll high on their hit dice and then make their saves. If you give the kobolds just the standard 4 hit points, the death toll gets much higher:
Wizard has Int 13 (minimum required to cast fireball): 1 in 926
Wizard has Int 16 (typical for a wizard built with the "elite array", with level 4 stat bump): 1 in 1,234
Wizard has Int 19 (typical for a point buy wizard, with level 4 stat bump): 1 in 1,481
Wizard has Int 25 (grey elf using point buy and fox's cunning): 1 in 3,703
Personally, I still would not bother rolling, because who the heck cares? It's a bunch of kobolds. If one of them does get lucky and survive, it'll just get decapitated the next round. It's not worth my or my players' time to game out a low-probability event with negligible consequences. Now, if it were a player character down to 4 hit points who was facing an NPC wizard's fireball, of course I would roll; the consequences there are far more significant.
Wizard has Int 13 (minimum required to cast fireball): 1 in 37
Wizard has Int 16 (typical for a wizard built with the "elite array", with level 4 stat bump): 1 in 49
Wizard has Int 19 (typical for a point buy wizard, with level 4 stat bump): 1 in 59
Wizard has Int 25 (grey elf using point buy and fox's cunning): 1 in 141
So, it's actually not as unlikely as I would have expected--mostly because of the possibility of extra-tough kobolds that roll high on their hit dice and then make their saves. If you give the kobolds just the standard 4 hit points, the death toll gets much higher:
Wizard has Int 13 (minimum required to cast fireball): 1 in 926
Wizard has Int 16 (typical for a wizard built with the "elite array", with level 4 stat bump): 1 in 1,234
Wizard has Int 19 (typical for a point buy wizard, with level 4 stat bump): 1 in 1,481
Wizard has Int 25 (grey elf using point buy and fox's cunning): 1 in 3,703
Personally, I still would not bother rolling, because who the heck cares? It's a bunch of kobolds. If one of them does get lucky and survive, it'll just get decapitated the next round. It's not worth my or my players' time to game out a low-probability event with negligible consequences. Now, if it were a player character down to 4 hit points who was facing an NPC wizard's fireball, of course I would roll; the consequences there are far more significant.