No way to adjust the quality. Streams are unschaubar on TV. It is apparently the lowest quality used and you can hardly recognize font. What shoud that? Now Amazon buys already Twitch and something comes out? Na Prost meal.
Update 11/25/2014:
Today I looked again into the app and it remains with the problems described above. Unfortunately, not yet touched up here. As a small tip, I have installed on my FireTV sideloading XBMC and use there developed by unpaid people and better times a thousand Plugin for Twitch.