It's not really popping – just burning with some extra drama! If you want your logs to burn safely without flying out of your fireplace, then stack them right-side-up so they can dry out. Firewood sounds like it's hissing or popping in the fireplace when there is moisture inside of it and then while it's being burned, water evaporates to create steam which creates noise that sounds like “hiss” or “shhhhhhh.” It's not really popping – just burning with some extra drama! If you want your logs to burn safely without flying out of your fireplace, then stack them right-side-up so they can dry out.