My 2599 is stuck in a boot loop. My understanding is that garmincure3 and the appropriate firmware may be able to fix it. The problem I am running in to is that you cannot download firmware gcd files from Garmin anymore, and I am having trouble locating a firmware file for my 2599.

gnufsh

Look here,my friend..

[Please Register or Login to download file]

Awesome, thank you!

How can I tell if I need the 25x9 firmware or the 25x9C4 firmware?

By checking the device's HWID. C4 devices have HWID of 1947 and "non-C4" have 1758. If you have a backup of the file system you can look in the Garmin folder for GarminDevice.xlm file and open it in a text reader or web browser. Look for this near the start:
<Model>
<PartNumber>006-BXXXX-00</PartNumber>
<SoftwareVersion>xxx</SoftwareVersion>
<Description>nuvi</Description>
</Model>
The four numerals represented by XXXX in red is your HWID.

If you don't have a backup you can read it's HWID via G7ToWin by putting it into preboot. G7ToWin is a free software, search for a download.

You may however be able to use Kunix's QuickCure3 rather than his GarminCure3 software as QC3 is not reliant on the actual device software, it's 'universal' and works with many soft-bricked Garmin devices to restore Mass Storage Mode. You will still need to access preboot of course which unfortunately is not easy with nuvi 25x9 and some other modern Garmin devices.