Sign [Q]ADB works, but fastboot won't recognize device. Can't update TWRP to 2.7 for kitka

Hello.

I have a KFHD 7" happily running under Hashcode's CM10.0. I'd like to update to kitkat.
For that, it seems I need to upgrade my recovery from TWRP 2.4 to 2.7. ( If the rom should install under TWRP 2.4, let me know, cause I got an error when I tried. )

So, after messing a bit with my drivers, I got ADB to work fine.

However, I need fastboot to update recovery. My Kindle can go into fastboot, no problem.

But windows just won't recognise it. "Fastboot devices" always show no devices connected.

I tried with various drivers ( including those from QEMU thread ) on 2 differents computers.

So I'm starting to wonder if there is something I missed in the virious threads. Is there something I should know that would prevent me from seing the device under fastboot but not ADB ?