This is because you have CEC enabled. It's a way for your TV to talk with a connected device over HDMI. You can disable this in your TV Settings.
All devices have built in harmony codes, meaning you don't have to do any pairing. So naturally, if you setup your harmony and flirc - firetv on one, the other will respond.
To remedy this, open up my software, go to file->advanced and disable the built in profile on the one you don't want to respond.
Don't believe it, I think your harmony is configured as noted above.