I can get every button on the firetv remote trained to my remote except for the back or move back one step button which on the firetv is just to the left of the home button.
I've tried on a second remote and same outcome.
p.s. plugged full keyboard into firetv and the esc key mimics the back key on the firetv remote. so using full keyboard gui of flirc i programmed a remote key to be the esc key. now i can get the go back a step function of the firetv remote. so leads me to believe that the firetv gui just needs that back key to be reprogrammed to be "esc".