Same experience here, Netflix works without too much hassle, but Prime is limited to a much lower bitrate than on Windows.

At one point I experimented a bit with making an addon myself, and it's possible to intercept the requests and spoof the UA/platform to get URLs for the HD streams, but then it goes to VMP/widevine and is a bit beyond my abilities. Best I could get was for the HD quality option to appear in the UI, but with a complaint that it wouldn't work because I needed to update or use a different browser.

Now pretty much the only time I use Prime video is for the occasional watchparty, and I'll just pirate the stuff I want to see in HD.