Socket.io cannot connect to server Heroku

some times you might get following error with your socket.io applications deployed on heroku. like

Unable to connect Socket.IO

error 
 or

Firefox can't establish a connection to the server at ws://*.herokuapp.com/socket.io/1/websocket/FGeqBo-pw7H3UCV97gg7.

this is because Currently Heroku cedar stack does not support websockets protocol so

change your socket.io setting's to following.

// assuming io is the Socket.IO server object
io.configure(function () { 
  io.set("transports", ["xhr-polling"]); 
  io.set("polling duration", 10); 
});