Get Tab URL

How to get the url for a tab?
My Tabs is Ajax:
<div id="tabs">
<ul>
<li><a href="content1.html">Tab1</a></li>
<li><a href="content2.html">Tab2</a></li>
</ul>
</div>
<button onclick="alert(????)"> Tab 1 (or index 0) URL</button>
Thanks!