If you have trouble with fastboot mode dropping out when going from MfgTool to linux fastboot, try setting fastboot mode from the diags exit menu (install fastboot package), which will make it always boot to fastboot until you change the bootmode var.
EDIT: Hey amazon! If you are reading this -- it really sucks when we have to clean up your mess when your updates brick kindles (especially kindles in foreign lands). You know? And your recent "anti-hacker" mods in 5.1.0 are only going to make it harder for us to fix your problems. Get a clue and give us a break! We are only trying to HELP people here. Okay?