The latest release of Firefox addresses 11 vulnerabilities that could potentially be exploited by an attacker to run arbitrary code while the user browses normally.
The issues are listed here.
New in Firefox 20 are private browsing, a revamped user interface for downloading, and the ability to close a hung plugin.
HTML5 support has been extended with the addition of support for camera and microphone, blend modes in <canvas>, and <audio> and <video> improvements.
Various performance improvements are also included.
A list of changes and known issues is available here.