Just to speak numbers:
FW400 = 400Mb/sec = 50MB/sec
FW800 = 800Mb/sec = 100MB/sec
USB 2.0 = 480Mb/sec = 60MB/sec
These numbers are theoretical, and many tests have proven that Firewire is more efficient at handling mass storage than USB 2.0.
And, even though the interface of the latest drives is 150MB/sec (for Serial ATA), 133 MB/sec (ATA-133), and 100MB/sec (ATA-100), the drive itself can only sustain a rough transfer rate of 40MB/sec.