Just the firmware and qemu won't help you much. When the firmware starts up, it will try to communicate with the hardware - initialize the network card, check which channels have cameras attached to them, probably read some configuration values from non-volatile storage, check the total/free space of the hard disk, this kind of stuff.
On a plain qemu emulation, the hardware just isn't there. So the firmware will probably send a command to the to read the MAC address from the network card (to determine its own serial number), then wait until the network card responds. But, the non-existant network card never responds. So if you're lucky, the firmware will throw an error message and abort the boot; if you aren't lucky, it will just enter an endless loop without you knowing what's happening.
Anyway, unless you a) write qemu modules to emulate everything the hardware of your dvr does, or b) disassemble the firmware, and patch all the device drivers to simulate hardware that isn't there, you won't be able to run your firmware with qemu.