The Synthiam ARC software (formerly known as EZ-Builder) can indeed be used with an Arduino Uno, including clones like the Elegoo Uno, which are hardware-compatible with the original Arduino Uno. To use the Arduino with ARC, you would typically flash the Arduino with the EZ-B firmware, which allows it to communicate with ARC.

As for using the new Arduino IDE 2, the process of flashing the firmware should be similar to using the previous version of the IDE. The important part is to ensure that you select the correct board and port before uploading the firmware to your Arduino.

Regarding the _FIRMWARE_ID value, you should not need to change this unless you are customizing the firmware for some specific purpose. The firmware ID is used by the software to identify the type of firmware running on the board, and changing it could cause compatibility issues.

If you are experiencing connection issues, here are a few steps to troubleshoot:

1. Check the COM Port: Make sure that the COM port selected in ARC matches the one assigned to your Arduino Uno by your computer's operating system.

2. Board and Port Selection: In the Arduino IDE, ensure that you have selected the correct board (Arduino Uno or equivalent) and the correct port before uploading the firmware.

3. Drivers: Ensure that the drivers for your Elegoo Uno are correctly installed. Clones sometimes require different drivers than the official Arduino boards.

4. Baud Rate: Verify that the baud rate specified in the ARC connection settings matches the baud rate expected by the firmware on the Arduino.

5. Reset the Board: Sometimes, simply resetting the Arduino board can resolve connection issues. You can do this by pressing the reset button on the board.

6. Check the Firmware: Make sure you have uploaded the correct firmware to the Arduino. If the upload was not successful, the board would not be able to communicate with ARC.

7. Firewall/Antivirus: Occasionally, firewall or antivirus software can block the connection. Check to see if your security software is preventing ARC from connecting to the Arduino.

If you continue to have issues, you can seek additional support by posting on the Synthiam community forums or checking their support articles for more detailed troubleshooting steps. Remember to provide as much detail as possible about the steps you've taken and the issues you're encountering to get the most helpful assistance.