What you would do is reboot to an ssh-enabled diags mode, then ssh into your Kindle and use "dd" to overwrite the main partition with the image from an earlier fw release.
Sounds fun?
OK, you asked for it:
http://ixtab.tk/kindle-touch-images/PW2/README.txt
This is the only known image for an actual fw5.4.0 -- Amazon never released an update for the very first fw revision.