AFAIK there is only one firmware version, the shipped one. And so there is no need to offer a download. When I got this box, I have searched for the firmware, to sift it to find a way to enable telnet. I couldn't find it that time, so I downloaded the GPL sources instead.
Rebuilding the firmware from the original GPL sources is a trivial task, which I could do for you if you want. However, there is no way to guarantee that it will give exact the same firmware.
You can backup your current firmware:
dd if=/dev/sxapp of=/path/to/backup
But I don't know if there is an (easy) way to put it back. Maybe just reverting:
dd if=/path/to/backup of=/dev/sxapp
maybe converting it to a motorola hexfile (make sure the addresses fit) and using sxflash
to flash it. (A great tool btw. It saved me several times while creating my custom firmware. Thanks Wladimir!)