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
• Press Home button by going to reboot option through Volume up button.
If this was a temporary problem, you could use your device now.
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.