FireWire

FireWire is Apple's trademark for the IEEE 1394 family of standards. FireWire 400 is "S400" IEEE 1394a and is capable of roughly 400 Mbit/sec and uses the rugged 6-pin socket (not the abominable 4-pin socket). FireWire 800 is "bilingual S800" IEEE 1394b-2002 and uses a 9-pin socket.
Source: Michael Johas Teener

Firewire was a popular bus in the pro-world to connect the AD/DA converter to the PC.

It offered a performance USB1 couldn’t match.

Using USB2 or 3, performance is not the issue anymore.
PC’s with a Firewire interface are becoming rare.
USB simply rules.
Even Apple, the one who made Firewire popular, has dropped it.

FireWire was phased out of Mac hardware between 2008 (when the MacBook Air released without a FireWire port) and 2012 (when the last FireWire-equipped Macs came out).

arstechnica

Interface

The TC Audio DICE chip is often used for the Firewire connection.

A lot of Firewire DACs use adaptive mode just like their USB counterparts.

Other than Metric Halo all the Firewire protocols work the same way adaptive does. That means any company using TC Electronics Firewire (From the Oxford set).
Only Metric Halo has an Async Firewire setup, everyone else is not.
J. Gordon Rankin

Jitter

On the audiophile forums they discuss jitter.
The proponent of Firewire claimed it to be jitter free.

It isn't.

Julian Dunn explains why.

Sample clock jitter and real-time audio over the IEEE1394 high performance serial bus

The Well-Tempered Computer

FireWire

Interface

Jitter