Mozilla released Firefox v21 today, the latest version of their Firefox browser.
The major improvements in Firefox v21 over v20 involve an update to the Social API (which you can read about here), enhancements to the Do Not Track (DNT) feature, and a new feature called The Firefox Health Report, which you can learn more about in Mozilla's blog post about it here.
You can download Firefox v21 here, or just watch for the update notification next time you're using the browser.
In addition to the new features I already mentioned there were a large number o bug fixes, all of which you can read on their Firefox v20.0 Bug Fixes page. A much easier-to-digest highlight of the new features and major fixes can be found here.