So far as I’m aware Firefox’s default for Alt and left/right arrows is to navigate page history backwards and forwards. I think the OP at least wants to use the codeacdemy IDE shortcut which suggests you can access the next lesson using Alt+>(or Ctrl+>) the symbol >, not the arrow key, once you have completed the tasks. Since that’s not in your page history I don’t think that’s what they’re after.
Apologies if that’s not the behaviour you’re describing.