Flock is a discontinued web browser that specialized in providing social networking and Web 2.0 facilities built into its user interface. Earlier versions of Flock used the Gecko HTML rendering engine by Mozilla. Version 2.6.2, released on January 27, 2011, was the last version based on Mozilla Firefox. Starting with version 3, Flock was based on Chromium and so used the WebKit rendering engine. Flock was available as a free download, and supported Microsoft Windows, Mac OS X and, at one time, Linux as well.