Welcome to the forum barbiegirl!
If you want a drive that can do a lot of things in both formats I would highly recommend the Sony DRU-500A. The review of this drive can be read here.
http://www.cdrlabs.com/reviews/index.php?reviewid=154
As for the firewire and USB2.0, there are some external versions of DVD burners that support both connections. You just have to look through the different products offered by the manufacturers. If you don't find an external model to your liking, you can always buy an internal one and put it into an external drive case that supports firewire and/or USB2.0