No, it would be possible the other way around potentially, but it is physically impossible to implement a generic firewire interface over usb with any kind of performance. Firewire needs direct system bus access which usb does not have, therefore it can not provide. More information on the technical reasons why it isn't possible are available on SuperUser.
There are some adapters out there, however they either use a special controller that can emulate a particular firewire mode (such as a firewire HD or generic DV video camera) and then translate the stream on the fly to a sufficiently fast USB connection. These adapters work reasonably well, but they are expensive, have limited functionality and can only be used for the particular type of device for which they were designed.
The other type of device is simpler and cheaper, but is effectively a rip off with limited real usefulness. It is possible to emulate system bus access by passing data over USB and then using the CPU itself to put the information on to the bus, but such behavior is time consuming and will be too slow for most purposes (possibly preventing the protocols on the line from even working at all).
Your only real option would be using a bus expansion card(ie. A pci card, express card or pcmcia card). Alternately thunderbolt, if you have it, is backwards compatible to firewire as it does have system bus access and can implement the entire subset of functionality required by a firewire interface.
Also, while not immediately helpful, last I checked it is still possible to get PCs with firewire ports built in though it substantially limits your options.