you have the same id for all your tabs. See fixed:
<ul class="tabs">
<li><a href="javascript:tabSwitch('tab_1', 'content_1');" id="tab_1" class="active">tab 1</a></li>
<li><a href="javascript:tabSwitch('tab_2', 'content_2');" id="tab_2">tab 2</a></li>
<li><a href="javascript:tabSwitch('tab_3', 'content_3');" id="tab_3">tab 3</a></li>
<li><a href="javascript:tabSwitch('tab_4', 'content_4');" id="tab_4">tab 4</a></li>
<li><a href="javascript:tabSwitch('tab_5', 'content_5');" id="tab_5">tab 5</a></li>
</ul>