Style-Changing Handler For a Select Box
Instead of testing the value of the HTML element, pass the season number in as a variable, and then you can use some of that simplifcation magic we used before:

function seasonChange() {

    var seasonNum = document.getElementById('selectseason').value;
    var first_episodes = [
        "1 - Death Has A Shadow",
        "1 - Peter, Peter, Caviar Eater",
        "1 - The Thin White Line"
    ];

    for (var i = 1; i &lt;= first_episodes.length; i++){
        if (document.getElementById('selectseason' + seasonNum + 'episode').value == first_episodes[i - 1]) {
            document.getElementById('season' + seasonNum + 'episode1').style.display = 'inline-block';
        }
        var showOrNot = (seasonNum == i ? 'block' : 'none');
        document.getElementById('season' + i + 'episodes').style.display = showOrNot;
        document.getElementById('s' + i + 'buttons').style.display = showOrNot;
    }

}


This uses the season numbers, to test whether its the one you're switching into or not.

One suggestion I'd make though is that, instead of the first_episodes variable, I'd use a database script, in PHP or something similar, to hit the database for the different episodes/seasons.

That way you can totally move this to the automated paradise you want.