What/where is what? The firmware update image? That can greatly depend on the device you have, but I've has lots of luck searching places like
http://freaktab.com and
http://forum.xda-developers.com for specific firmware. I'm sorry if I came off as rude, it's just frustration with many of the AMLogic-based box sellers who use funky firmware images.
Firmware is basically an OS install, Android itself. Firmware is typically specific to a certain set of hardware as well.
There are some files that XBMC needs access to in order to use hardware decoding in v13, which uses a different method than what was used in the v12 AMLogic builds (AMLplayer vs AMLcodec). If he permissions aren't correctly set for those files then XBMC is denied access, and then can't decode the video.
This can be fixed in one of two ways. One is to just reinstall Android using a better firmware. The other is to manually set permissions on those files (off hand, I don't know which and I'm not sure which command it is, but it had been posted about on the forums a couple of times).