This my be a stupid question, but can I burn a video file from my computer to my Panasonic DMR-ES15 via firewire? And if possible, how would I do it?

You could if you had an application that ran on the PC that simulated a camcorder. And of course the file you are trying to transfer must be in the DV format.

Offhand I don't know of such an application, but this sort of thing is discussed over at VideoHelp.com often. There is even a DV/HDV Forum there.