How are external FireWire burners better?

Just slightly better speed, right?

Yes, the only thing about firewire is that it’s a little bit faster but not by much. Each has its own advantages. But as far as Firewire 400 VS USB 2.0 you will not notice much of a difference. But comparing Firewire 800 to 400 and USB 2.0, the transfer speed on Firewire 800 is 320Mbps faster than USB 2.0. But regardless of that, both Firewire 400 and USB 2.0 need external power to burn anyways unless you are using a DVDRW Laptop slim drive as an external burner as this is where USB might have an advantage.

Firewire is always better than USB etc. because it has not their weaknesses and flaws, eg. powering.

Firewire will estrablish a continuos stream of data (both ways) and will not depend on I/O as USB does (this gives you the idea of continuity via buffer and is more processor dependent).
About speed, don’t forget USB devices share the controller, so 480 will be shared if you have several, and if one of them is version 1.1 will most probably slow down the others.