firefox sockjs nginx error

[mibto]

Hi there :)
I have a nginx reverse proxy before the meteor app. I get the following error only in firefox:
Firefox kann keine Verbindung zu dem Server unter wss://mydomain.com/sockjs/876/dx24wg4k/websocket aufbauen.
(Firefox can not connect to the server under wss://....)
i saw this error also on atmospherejs.com.

my nginx config is:

location / {
        proxy_pass                      http://upstream;
        proxy_http_version              1.1;
        proxy_buffering                 off;
        proxy_socket_keepalive          on;
        proxy_set_header                Host $host;
        proxy_set_header                X-Real-IP $remote_addr;
        proxy_set_header                X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header                X-Forwarded-Proto $scheme;
        proxy_set_header                X-Forwarded-Port $server_port;
        proxy_set_header                Upgrade $http_upgrade;
        proxy_set_header                Connection $connection_upgrade;
        proxy_ssl_server_name           on;
}

any idea?

Thanks
Michael