Most of the Fire tablets can have an SD card inserted to increase storage of AMAZON CONTENT.
(None of the eReader devices can or need memory expansion.)
Note: The SD card will only store Amazon content - books, movies, some apps, that kind of thing.
If you are having storage problems with a fire tablet it's probably time to consider Restoring Factory Defaults - it will wipe data, game scores and in-game app purchases if the apps do not use Game Circle. It will make your Fire perform like the day it arrived.
As the Fires use Android with Amazon's own interface it would be possible for them to include "adoptive" memory (dependent on the version of Android) which would allow the SD card (although now locked to the device) to act as system memory if you wish. Problem solved.