When I press run in the code editor, the code executes, the cursor in the repl.it console moves but nothing is visible when using Firefox 68.0.1. I disabled content and ad blocking with no change. The same effect with either dark or light theme. Everything works correctly when I run Repl.it in Vivaldi.
Works for me in FF 68.0.1 - can you link me to the repl just to be sure we're trying the same thing?