Unless enabled, encryption keys can be easily recovered by an attacker with physical access to any machine with PCMCIA, ExpressCard, ThunderBolt or FireWire port. More information is available at https://en.wikipedia.org/wiki/DMA_attack.
This option blacklists FireWire drivers, but doesn't remove them. You can manually load the drivers if you need to use a FireWire device, but don't forget to unload them!