$("table.SimpleTable tbody tr:visible td:first-child");

<table class='SimpleTable'>
  <tr style='display: none;'>
    <td>test1</td>
  </tr>
  <tr>
    <td>test2</td>
  </tr>
  <tr>
    <td>test3</td>
  </tr>
  <tr style='display: none;'>
    <td>test4</td>
  </tr>
  <tr>
    <td>test5</td>
  </tr>
  <tr>
    <td>test6</td>
  </tr>
</table>

$("table.SimpleTable tbody tr:visible td:first-child").css('color','red');

var serials = [];
$("table.SimpleTable tbody tr:visible td:first-child").each(function() {
    serials.push($.trim($(this).text()));
});
var serials = serials.join(',');

$("table.SimpleTable tbody tr:not(:hidden) td:first-child")

var notfirst = false;
    var serials = "";
    var tds = $("table.SimpleTable tbody tr:visible td:first-child");
    for (var i = 0; i < tds.length; i++) {
        var td = $(tds[i]);
        if (td.is(":hidden"))
            continue;
        if (notfirst)
            serials += ",";
        else
            notfirst = true;

        serials += $.trim(td.text());
    }

$("table.SimpleTable").find("tr:visible").find("td:first-child").text()