Connecting a FireBeetle ESP32 to Arduino
The flash button is virtual and controlled by the RTS signal from the computer through the USB port (why are you trying to connect through an Arduino?). It should automatically enter bootloader mode when you go to program it. There should be nothing you need to do.

However, if you do want to manually enter bootloader mode, just connect IO0 (note: not D0) to GND and press the reset button.