In Cronological order serial came first,USB then Firewire.
They are, as you no dought know, just differant interfaces/connections to your system.
I carn't give you the specs off the top of my head but in short: Serial/and parrallel are old hat now, slow, and should be avoided if you have a choice.
USB is the most common, very user friendly and the most widely supported, also much faster than serial/parallel
Firewire is the new boy! ultra fast! leaves USB for dust but is not as widely available and it's benefits are only felt when large files and data are needed to be transfered fast!ie: video editing
Whatch out for an improved USB connection, much quicker than the original and very creatively called USB2
No dought someone will be kind enough to post you the comparative transfer rates between them all.
Just remember the quicker Firewire won't be any faster if the connected hardware doesn't use it's potential.
At the moment buying firewire periferals will probably cost you more. Great if you need the speed! Martin Vote if you found this post helpful please!!