[Unseen Entities]

Bear with me here, the timeline is confusing. Based on jhowell's quoted post below and current firmware versions from Amazon it seems like If you want KF8 compatibility on a kindle that currently doesnt support KFX your choices are a Kindle Touch (2011) and PW1 (2012) ...or perhaps any device gen 3 through 5 according to the KF8 wiki page.

I'm really not sure how comparable the firmware versions across devices are. I saw like decade old threads talking about a KF8 update on Kindle Keyboard but more recent ones asking when the Kindle Keyboard is going to get KF8...

Older than 3rd Gen and you lose WiFi. Without KF8 support youre working with MOBI. Kindles after the PW1 need a lower-than-current firmware versions, or jail-breaking to make them to downgrade I guess. Dont know if anything past the PW3 launched with firmware under 5.12.2. No telling if or when Amazon decides to push an update that switches older devices to KFX.

Originally Posted by jhowell

If is has firmware prior to version 5.6.5 it does not support KFX. If the firmware is prior to 5.12.2 it supports KFX but DRM removal is possible.

Amazon - Kindle E-Reader Software Updates (current version numbers)

3.4.3    - Kindle Keyboard 3rd Gen
4.1.4    - Kindle 4th and 5th Gen
5.3.7.3  - Kindle Touch 4th Gen
5.6.1.1  - Paperwhite 5th Gen aka PW1
5.12.2.2 - Paperwhite 6th Gen aka PW2 
5.12.2.2 - Kindle 7th Gen aka Kindle Basic
5.13.6   - Kindle Voyage 7th Gen
5.15.1   - Paperwhite 7th Gen aka PW3

Amazon Kindle - Wikipedia

3rd Gen -    August 2010 - Kindle WiFi aka Kindle Keyboard
4th Gen - September 2011 - Kindle 4
4th Gen - September 2011 - Kindle Touch
5th Gen - September 2012 - Kindle 5
5th Gen -   October 2012 - Kindle paperwhite 1
6th Gen - September 2013 - Kindle paperwhite 2
7th Gen -   October 2014 - Kindle 7 aka Kindle Basic
7th Gen -  November 2014 - Kindle Voyage
7th Gen -      June 2015 - Kindle paperwhite 3

The MobileRead wiki is great btw, but it seems like its missing a little info. For example it doesnt mention KF8 compatibility on the individual pages for the Kindle Keyboard, 4, 5, and Touch but then says they all have KF8 compatibility on the KF8 page minus 1st/2nd and DX.