While the official release announcement has not yet come down, as usual, the early binaries for the next Firefox release are now available. The Mozilla Firefox 21.0 release that's now available from FTP servers
features numerous new features.
The release of Firefox 21.0 tomorrow will advertise its Do-Not-Track improvements, initial work on the FIrefox Health Report, CSS improvements, graphics-related performance improvements, remote profiling support for developers, and HTML5 support of the "main" element tag and scoped stylesheets.
At the time of writing, Firefox 21 for Android hasn't yet been released but only the binaries for Linux, OS X, and Windows. Firefox 21 for Android will feature similar improvements plus new font support for web content, support to save media files on long tap, and other mobile enhancements.