How to fix a phone stuck at firmware update screen? My phone was dead so I plugged it in to charge. I attempted to turn it on and saw:

Firmware Update
Do not unplug the USB connection until the process is complete.

This was all it showed and now it's frozen. It does not power on or off. It's been stuck at 6% for 8 hrs now. I have no battery to take out.
What can I do?
It may be an update to your device, but it sounds unusual because no device is allowed to update its firmware when the device battery is running below 30% anyhow, unless you clicked on Update Now button that time, and plugged it into charging. 


If it's an update, wait some time as you told your device is dead that means system has downloaded that file before turned off. So it will automatically install its firmware.
In the worst case, you may need to install a very new firmware manually but before continue to install new firmware try this:

• Press Power + Volume up + Home simultaneously until Samsung logo appears. 

• It may take time to show up recovery menu. In recovery menu you will see option as Reboot.

• Press Home button by going to reboot option through Volume up button.


If this was a temporary problem, you could use your device now.

Or

If the screen is still stuck to showing Firmware Update, the solution could be to update it by yourself.
To update go through this article: 
How to update the firmware on any Samsung phone? by xda-developers.

Please keep in mind, manually updating Firmware may erase your whole data.