Protocol upgrade mechanism - HTTP
sec-websocket-extensions specifies one or more protocol-level websocket extensions to ask the server to use.
... sec-websocket-extensions: extensions extensions a comma-separated list of extensions to request (or agree to support).
... for example: sec-websocket-extensions: superspeed, colormode; depth=16 sec-websocket-key provides information to the server which is needed in order to confirm that the client is entitled to request an upgrade to websocket.

nsIWebSocketChannel
nel); method overview void asyncopen(in nsiuri auri, in acstring aorigin, in nsiwebsocketlistener alistener, in nsisupports acontext); void close(in unsigned short acode, in autf8string areason); void sendbinarymsg(in acstring amsg); void sendmsg(in autf8string amsg); attributes attribute type description extensions acstring sec-websocket-extensions response header value.

HTTP headers - HTTP
sec-websocket-extensions ...