Firewire has its own controller chip on the logic board, and as the MBA and MB have no Firewire, this chip is missing, thus not even a magical hub will convert from USB to Firewire or vice versa.
USB goes via the CPU.
You can take a look at the dozens of similar threads about the same issue, as it arose when the 13" Unibody MacBook was introduced back in 10/2008 and it was missing a Firewire port.
MRoogle is a good tool to search these fora for already existing threads about questions you have. It might be able to answer you quicker than waiting for an answer.