With mobGriefing gamerule set to false, fireballs still set fire to surrounding blocks.

In vanilla, with mobgriefing = false, fireballs do not set fire to surrounding blocks.

To reproduce:

/gamerule mobGriefing false

Fly up into the air and...

/summon fireball ~ ~-1 ~ {ExplosionPower:10,direction:[0.0,-5.0,0.0]}

The ground below you should now be lit, high explosion power to reliably set fire.