The configuration file should be named throwable-fireballs.toml and should be written by default:

{
"fireballType": "blaze",
"cooldown": 0
}

"fireballType" - is responsible for the type of fireballs. The mod supports fireballs of three mobs: blaze, dragon and ghast.

"cooldown" - is responsible for the cooldown in ticks. Just like the ender pearl.

You can also change the settings with the command /fireballcfg.