Custom third-party ROMs provide a good way to keep older hardware running current versions of Android. And in this case the aging hardware we are talking about is the Nook Tablet and the original Kindle Fire. These were both originally released with a customized version of Android 2.3, however thanks to CyanogenMod they can be running Android 4.3.
In both cases the ROM you would need to look for is CyanogenMod 10.2. Builds for both of these tablets can be found by way of the xda-developers forums (Nook Tablet and Kindle Fire) and both can be paired up with an Android 4.3 build of Gapps, which is what you will need to get all the Google apps such as the Play Store.
Similar to installing CyanogenMod on any other tablet or smartphone, there is a bit of work to do in order to get up and running. This process isn't necessarily hard to do, but it should be done with caution as things can go wrong if you make some mistakes. While the process will vary from device to device, the basics is that you will need to root the tablet, install a custom recovery and then flash the ROM (followed by Gapps).
Of course, those who have already rooted their tablet and installed a custom recovery will have it a bit easier. Looking back and we have seen other CyanogenMod builds for these two tablets. For example, a build of CM10.1 (Android 4.2) came available for the Nook Tablet back in December of last year. The upgrade from CM10.1 to 10.2 may not be as big, but those going from the stock setup to CM10.2 will see more than a handful of changes.
That all being said, aside from the warning about making sure you proceed with caution in terms of updating these devices -- it should also be said that CM10.2 is still under development and these builds are both based off the nightlies. Or in other words, those updating to CM10.2 may experience some bugs or other issues.