FireWire connections come in two types: a four-pin connector and a six-pin connector. On Windows-based systems, laptops with FireWire connections are usually four pin, as it easier to get the smaller jack into the available real estate. Most Mac systems have a six-pin connector. Six pin FireWire connectors are capable of powering units off the computer's bus. If your computer is equipped with six-pin and four-pin inputs, get a six-pin based setup so that you can power the unit off your computer in the field, where AC may not be available.
When researching an audio interface for your DAW, make sure it is capable of recording at 24 bits and a sample rate of 96k, often referred to shorthand as 24/96. CDs are in 16/44.1 format, so you might question why 24/96 is necessary. However, 24/96 recording gives you more headroom for things like level peaks, and the faster sample rate more accurately recreates the analog waveform. Professional studios always record at 24/96.
The inputs and outputs on any FireWire recording device will determine its usefulness. Some boxes only have one input, usually an XLR microphone connection with a microphone preamp and a 1/4 inch TRS plug. More advance boxes have multiple channels with XLR and TRS connections, MIDI connections, and even left/right RCA inputs and even digital coaxial inputs to connect to stereo systems. Some will also have multiple outputs to power outboard studio monitors, a pair of headphones, and even an effects loop. It is important to determine how many inputs you will need for your recording project before purchasing a FireWire device.
Some FireWire recording devices will come packaged with recording software that you can use. If you already own a recording software package, such as Cubase, SONAR or Pro Tools, check the manufacturer's website to make sure the FireWire device has drivers that are compatible with your recording software. If you are intending to record for later production in the studio and don't have software yet, look for a FireWire unit that comes with Pro Tools LE, a light version of the full Pro Tools used by most professional studios. All versions of Pro Tools are designed so that Pro Tools HD, favored by most studios, can import recordings made with Pro Tools LE or M-Powered Pro Tools.