Mozilla Fennec 4.0b2 for the "linux-i686" platform. The thumbnail with an anti-clockwise arrow ↺ represents a closed tab, which can be reopened with a click on that thumbnail.
Screenshot: <a href="//commons.wikimedia.org/wiki/User:AVRS" title="User:AVRS">AVRS</a>
<ul><li>Window manager: authors of Xfce</li>
<li>Mozilla Fennec: the contributors</li></ul>
MPL 1.1
own screenshot