...
//from flex client
NetConnection nc = new NetConnection();
nc.connect("rtmp://localhost/firstApp"); - connected, works

//from libRTMP

RTMP* r = RTMP_Alloc();
retVal = RTMP_SetupURL(r, "rtmp://localhost/firstApp");
retVal = RTMP_Connect(r, NULL); - no connection

retVal = RTMP_SetupURL(r, "rtmp://localhost:1935/firstApp");
retVal = RTMP_Connect(r, NULL); - still no connection

//The same thing works fine for Red5 "live" application

retVal = RTMP_SetupURL(r, "rtmp://localhost:1935/live/instance");
retVal = RTMP_Connect(r, NULL); - works, connected