Wow strange. I just found this on one of my systems.
I just installed a new 500 GB 7200RPM drive into my MacBook Pro. I then installed and patched Snow Leopard.
I then connected the old drive via USB and fired up Migration Assistant. It shows the user "firebird." And beneath the name in small type it says "This user's data doesn't need to be transferred."
Real weird I've never seen that user before.