Due to security issues WebSockets support has been disabled by Mozilla in Firefox 4 we’ve covered how to enable WebSockets for Firefox 4, looks like latest Aurora channel update added WebSockets support back to Firefox 6 (not for Firefox 5 mind  you).

Read How to Switch between Firefox Update Channels

WebSockets allows interactive communication between a browser and a Server. WebSockets used to build HTML5 chats , multiplayer games and more.

You can read more details about Aurora 6 update here.