If that Kindle still gets seen as a removable drive when you connect it to a computer using USB cable you should use your computer to create a file with filename "DO_FACTORY_RESTORE" (so file shouldn't have
any extensions, if you're creating the file on Windows using Explorer it will add ".txt" extension so you'll need to use rename to remove the extension) > eject Kindle from you computer > unplug USB cable > press and hold power button on Kindle for 20-30 seconds / until it force reboots.
That should trigger a factory reset and after that hopefully your firmware downgraded Kindle should be functional again. After that make sure to reinstall jailbreak hotfix and restart / reboot your Kidle 2-3 times.
Here you go, to make sure you don't mess-up creating that extensionless file I've done it for you, just unzip / unpack that zip file into the "root" ( = top level) level of Kindle drive:
unzip_me.zip