First read the article about how to access the firewall through serial console.
Then follow these steps:

1. During boot you will see a menu with title GNU GRUB. Press a cursor button in order to stop the countdown. Otherwise it will boot automatically.
2. Select the first menu entry and press 'a'
3. You should see something like the following:
   
   grub append> root=/dev/md3 panic=10 ro
   
4. Append 'console=ttyS0,38400' to the end of the line, in order to finally have a line like this:
   
   grub append> root=/dev/md3 panic=10 ro console=ttyS0,38400
   
   
5. Press ENTER, which starts the boot process