Amazon has stated the USB port on the Fire TV Stick is “for power only”. That’s not entirely accurate. I’ve discovered that connecting to the Fire TV Stick via ADB using a USB cable is possible and fully functional. Simply plug the Fire TV Stick’s USB “power” cable into a Mac or Linux computer, open a terminal window, run the command adb start-server, and then run the command adb devices to see that the Fire TV Stick is detected as a connected device. I was able to sideload apps and navigate the file system as you would expect with any ADB connected device.
Due to the lack of drivers, I was not able to make this work with a Windows computer. I tried installing Amazon’s Fire Drivers, but still had no luck. Currently there isn’t much you can do via ADB over USB that you can’t already do via ADB over Wifi, but this does confirm the USB port is alive. Having a functional USB port may lead to future abilities that have yet to be uncovered.
Thanks to a tip in the comments from ges, Windows users can install MoboRobo, an Android Phone Manager, in order to get drivers for the Fire TV Stick. Install MoboRobo, then plug in your Fire TV Stick and it will detect and install drivers. Be sure to exit MoboRobo before trying to connect to the Fire TV Stick via ADB. You can also uninstall MoboRobo once your Fire TV Stick is working in Windows and the drivers will remain. Note that MoboRobo will install some junk apps on your Fire TV Stick.