[Old Pedant]

Well, you have zero code in there doing that, so far as I can see. So I haven't the foggiest idea why it works in Chrome.

You could, however, easily fix the code:

    function loadNextPage(dir) {   
        cnt += dir;
        // this can never happen: if (cnt<0) cnt=webpageArray.length-1; // wrap
        
        if (cnt>= webpageArray.length) {
            return true;
        } 
        var iframe = document.getElementById("myframe"); 
        iframe.src = webpageArray[cnt]; 
        countDown = 10;
        doCountDown( );
        $('html, body').animate({ scrollTop: 0 }, 'fast');
        return false;
    }

And then:

<input type="submit" name="submit4" value="Submit" onclick="return loadNextPage(1);">