The Social API is designed to allow social providers to integrate directly with Firefox, displaying selected content on the browser's sidebar or toolbar.
With the update Mozilla has added several new partners, including Cliqz, Mixi, and msnNow. Cliqz and msnNow are news aggregation services, while Mixi is Japan's largest social network.
Firefox 21 brings an enhanced UI for the Do Not Track Feature and preliminary implementation of the Firefox Health Report, which is a system that is designed to log browser health information like start up time, total running time, and number of crashes. It is designed to monitor browser performance and provide tools to fix potential problems. The update also includes a number of small changes and bug fixes, which can be found in the release notes.
Firefox 21 for the Mac is available for download from the Mozilla website.