What ISP?

Sorry, Vodafone ADSL

I wonder if I should factory reset my Amazon Fire TV just to make sure...

Maybe just try clearing the data before a reset. I cleared the Netflix cache a couple of times, but I wish I'd thought to clear data as well. 

There are two options under the app settings deep in the system menu for each individual app; One for clear cache & the other for clear data. The latter requires re-entering username & password when the app is restarted. 

Can confirm I am getting the exact same as you - basically the Fire TV stick never reaches HD whilst my Chromecast 2 does straight away. Tried a factory reset but this didn't fix it. Although I don't use it much anymore will be interesting to see if you have a fix for it.

Any luck with after a reset?

I should mention, my FireTV is rooted and has been sitting unused in a box without firmware and app upgrades for quite some time now. I think I'm still on firmware 51.1.3.0_513011520.

I didn't reset it yet but did clear app data and cache, same results. Tested with DNS4Me and Unotelly, same results. Tested with Fritz!Box, DLink and Audiocodes, same results. Tested on 2degrees fibre 200 and Vodafone cable 100, same results. Tested on ethernet and 5GHz WiFi, same results.

2degrees and Vodafone use different upstream providers. So the only two things I can think are either the box itself, or Amazon Fire TV apps use their own server/proxy provided by Amazon and that's having problems with unblock services.

I will reset it tonight...

May be worth trying a US VPN server for comparison to mitigate any DNS routing trickery at either end?

I'm having same issues with fire tv, on unotelly.

Freitasm, can you play something on Amazon (not netflix), press the home button and go back to it and play again, do you get HD for first few seconds sometimes 10 seconds.  i'm getting HD on Sopranos every time I exit then go back, after few seconds the quality goes low.

when press the pause the button the HD it highlighted, then it goes away.  sometimes the quality is really bad, then it catches back up but never makes it to HD again.

Yes, exactly like that. I have tested on WiFi again and Netflix seems to go up to 5000 kbps and play HD, if I plug the ethernet cable it goes down to 325 kbps again.

Waiting for the new Amazon Fire TV update to see if things improve, otherwise might leave on WiFi for now.

Did some testing last nite on my Fire stick and found that some programs started off in HD  for about 10 seconds and then  returned to HD after about 2 minutes.  Some stayed HD for a while then went back to SD and after 5 minutes I gave up on them.

I also found that I couldn't buy any PPV programs as my CC was issued in NZ and not the US.   Not really going to be a problem if the content is going to be SD.. 

Netflix was fine.  Also after the Unotelly routing change a couple of weeks ago  I can't get most of the channels on Amazon any longer.  i.e.  CBS Sports.  Access the menus  but no playing of content  with an internet (region) error message.  The same channel was fine on my Apple TV 3 with the same Unotelly DNS .