This is more likely to get noticed by somebody who can help, if posted in the Kindle Developer's Corner forum.
There is a Master Index sticky thread there, which points to a Master Index wiki for kindles. Perhaps there is somebody more knowledgeable in that forum, ESPECIALLY about the DX upgrade to firmware 3.x. The threads (indexed in the DX wiki page) should have instructions about installing and UNINSTALLING a firmware upgrade for the DX.
If all else fails, you should be able to downgrade the firmware using the internal serial port, but if your DX is jailbroken (or it you install a jailbreak) it should be much easier to replace the firmware over the USB cable.
Go to the Dev Corner and look around (especially in the DX Index -- see the Master Index sticky thread)...
Okay, I will grab some links for you... just a moment...
Kindle Developer's Corner Master Index (aka. Where Do I Start?)
DX(G) Index
Kindle 3.1 software for Kindle 2, 2i, & DX (under construction)
How to update Kindle 2/DX/DXG with YifanLu's firmware from 3.2.1 to 3.4.2
Kindle DXG B009 Restore firmware???
[SOLVED] Unbrick a Kindle DX Graphite
The thing is, part of the upgrade process includes making a backup image set so you CAN restore old version 2.5.8. Without that, you will need to get a copy elsewhere. I am not sure if anybody posted DX firmware images after
twobob removed his shared images (which contained a partition with private user data, which should NOT have been shared, leading to his wife's kindle getting blacklisted by amazon). However, it is safe to share ALL but that one little partition...
If you can download a copy of the required 2.5.8 files, or make a backup from another kindle DX, then you can follow the instructions in those threads to restore your firmware to 2.5.8. Then you should be able to follow amazon instructions to upgrade to the latest amazon-supported firmware.