There are more than one reasons to prefer 5.8.11...
The 5.9.x firmwares broke many, many things, especially for us jailbreakers. So, e.g., the support of active documents is finally gone.
(I now refer to the so called "kindlets", these *.azw2 files that could be used directly without having to launch KUAL or even install it first - e.g. VlasovSoft's CoolReader port, and KindCalc.) Also, the internal database structure is different.
From my point of view, 5.8.11 should be regarded as the ultimate (last "really usable") fw for all newer Kindle devices (except the KOA2, of course...). And surprisingly, this version even added a valuable real new feature beside some minor bugfixes: The new, much finer boldness and fontsize settings. This was why even I decided to upgrade from 5.8.7.0.1 to 5.8.11 in December.