I have an external drive that has one of those mini-USB ports on it, so I can plug it in to my MacBook Pro's USB port. I want to plug it into a IEEE 1394 port. Looking for an adapter online, I found dozens of IEEE 1394 Female to USB Male, but couldn't find any USB Female to IEEE 1394 Male. I'll probably just get a IEEE 1394 enclosure for the drive, but I was curious why I found so many of one and not the other? Is there some technical hardware problem preventing USB Female to FireWire Male? There must be at least some consumer demand for it if the reverse is so easy to find.
Besides specifying the gender (male or female), you need to properly specify the type/style of USB connector (A or B). Presumably you are looking for a "female Standard A".
I can think of two possible reasons for the scarcity of a suitable converter or adapter.
Even if you found the "adapter" with the connections you are looking for, that would not mean that the external drive would work with the MacBook. Numerous reviews of one FireWire-to-USB adapter complain of "it doesn't work". These are not simple interfaces, that if you swap in the proper connector or cable type, things will work. Rather these are interconnections that involve compatibility of the electrical power, signal interfaces and the data & control protocol layer at both ends.