[Raboon]

Amazon have updated the stick such that the AV sync is now back on their own ecosystem - Prime videos, apps and channels. Good work lads.

They've also fixed the audio dropout occurring around 40 minutes in / repeating at 80 mins etc.

In Kodi 17, there are audio dropouts when switching between videos with different frame rates. Playing a 23.98fps video then a 29.97fps one causes the audio on the 29.97 video to dropout once or twice every second. Kodi needs to be fully quit to fix, then the 29.97 title plays back as expected.
Respeed, resample, sync, sall the stuff that meddles with AV in Kodi is off and switch display rate is off too.

In Kodi 17, AC3 audio and DTS audio have a different offset time to maintain sync with the video. AC3 is around .2 behind, DTS slightly ahead. I've set Kodi's delay for the AC3 and need to mess around in settings to re-sync the audio on DTS titles.

So things are watchable but honestly it is not at all smooth a couple of months in on the new, improved FireTV stick - Either needing to restart Kodi or muck about with settings to watch a video.