Haven't been able to use my Kindle with Linux for a while anyway. I regret buying the device.
Are there jailbreaks and/or custom ROMs for Kindles? It's a nice little device; would be a lot nicer without it constantly phoning home to Amazon.
Yes! Jailbreak thread: https://www.mobileread.com/forums/showthread.php?t=186645 (Note that in some cases it requires opening the plastic and ordering some cable off of Amazon.)
The only custom ROM I know about that supports Kindle devices is KOReader (https://github.com/koreader/koreader). I don't like its interface one bit, but feature-wise it gets the job done just fine.
There's also Plato (https://github.com/baskerville/plato) for Kobo devices, though KOReader works there as well. In my experience jailbreaking a Kobo device is much less painful.