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There are two ways to pair an Echo Dot to a firestick.
The first way doesn't seem to work. In the Alexa app, when you go to devices and select your Echo Dot, it says you can link it to an echo remote. When you do this, it says to hold the play button for 5 seconds and this does not work. I've tried a hundred times by now.
The second way does work. In the Amazon app, go to More -> Settings. Scroll down to TV and Video. Select FireTV. Select Link or Manage Devices. I already have 3 dots linked to 3 different firesticks, so my options here may be a little different than yours. From here I select to "link another device". If it is your first link, it might just say "link device". Click to link another device. It will have you first select the Firestick you want to pair. Then it will show you a list of devices where you will select your Echo Dot.
That's it!
Now your Echo Dot will accept voice commands to control your FireTV.