OK, now that I'm home and can look at my Harmony remote settings, I find it is not the inter-key delay that I tweaked to make it so I did not get doubled up button presses (sorry about my bad advice above). Using the MyHarmony software, I chose the device I use to control the FireTV, selected Change Device Settings, selected the Device Command Repeats menu item...
...and set Device Command Repeats to 0...
I hope this helps you too.
-Randy