You should be able to find out whether a box supports firewire or not, either from the labeling or by referring to the documentation with it. If the vendor doesn't want you to open the box for the manual, have THEM open it; it's all to get a sale, so they should be happy to help you out.
All of the reasonably priced firewire boxes I've seen have been from "nonbrands." Certainly not from a brand name I recognize... Sorry I can't help you there.