Few details have been released concerning the update, but I can see that my Kindle Fire HD (2013) received one, and I have also been told that the Kindle Fire HDX has also been updated.
Amazon says that the update "includes general improvements and performance enhancements", but it is my guess that the update actually added new features which support the Fire TV.
Amazon's media streaming device launched in NYC yesterday, and one of its more interesting features is that you can stream content from a Kindle Fire HDX to the Fire TV. I think support for that feature was added to the Kindle Fire HDX earlier this week, but of course that is merely speculation.
On a related note, I can also see that the older Kindle Fire HD tablets also received a vague update, but I don't have any information on when that update happened or whether that update added similar support for the Fire TV.
Does anyone know the last time the Kindle Fire HD (2012) was updated?