A relatively primitive grenade, the fire bomb is normally a
breakable canister of flammable liquid with a fuse made from
cloth or other material that the liquid can soak into. Once the
fuse is set aflame, the bomb is hurled at the enemy; the canister
breaks open on impact to release the now burning liquid.
Yet the rules say this can be launched out of a grenade launcher, while the description makes it sound a lot closer to a molotov cocktail (as well as the price).
And in the Inquisitor's Handbook, you have Infermal Thermal Grenades
These devices explode with a shower of molten metal as
their incendiary fuel ignites. They are not as powerful as a
melta-bomb and thus cannot burn though most armour, but
are useful for melting down simple barricades and doors or
for starting fires. These grenades can be purchased to be used
either by hand (thrown) or in grenade or RPG launchers.
They can also be found in missile form.
These have a smaller blast radius, slightly heavier but the main difference is the price of 60 thrones, versus the 5 thrones of a firebomb.
It just seems very perplexing how one weapon is so good and cheap and outperforms the "IG Version" with RaW, especially when one is a molotov cocktail and the other is an actual military incendiary device.