Mozilla has released Firefox 20 'Aurora' so that we can test new features of the upcoming version of Firefox. There are 3 main features in Firefox 20:
New Private Browsing: Firefox's "porn mode" now launches in a separate window, instead of giving up the regular browsing session.
New Download Manager: The new panel-based download manager displays the current state of the download, offers access to the list of recent operations and provides an estimate for completing the job.
Hanging Plugins: Firefox 20 can close hanging plug-ins without crashing the browser
You can download Firefox 20 'Aurora' but remember to not to use it as your main browser. Mozilla Aurora channel is not stable and is for testing purpose.