Perhaps the new firmware deletes the developer certs, like recent firmware also deletes SSH dropbear files? VERY unfriendly of amazon to do that, if true...
The "dev certs" you seek are in the Kindlet Index (combined developer keystore).
The Kindlet Index is in the Master Index sticky thread.
If I give you a direct link, that would be "cheating", removing the whole reason for even bothering to create the master index and index wikis.
Perhaps the "dev keystore" link needs a "dev certs" comment to help find it?