jQuery Promise / Deferred / AJAX methods in sequence

Tabs.js

var Tab = function() {
  /*
   * EXTRAS
   */
  var Extras = function() {
    var show = function( id ) {
      var extra   = $( "#" + id ),
          d       = $.Deferred();
          chained = d;

      // If the Extra is a placeholder, gets its content
      var div = extra.find( "div:first" );
      if ( div.length )
        chained = chained.pipe( prepareExtra( div ) );

      // Authenticates the user if:
      // * the extra requires auth
      // * he is not already authenticated
      if ( extra.attr( "requires-auth" ) != undefined && !config.user_is_authenticated )
        chained = chained.pipe( authenticate );

      // When the two optional methods are resolved, show the content
      chained.done( function() { extra.fadeIn( 500 ) } );
      
      d.resolve();
    };

    var prepareExtra = function( div ) {
      return $.post( div.attr( "href" ), {}, function( data ) {
        div.html( data )
      } );
    };
  }();

  var authenticate = function() {
    var d = $.Deferred();
    FB.ui( { method: "permissions.request", perms: "email" }, function( response ) {
      if ( response.session )
        $.post( "/authenticated/create_or_log_in_user", { 
          access_token : response.session.access_token
        }, function() {
          config.user_is_authenticated = true;
          d.resolve();
        } ); } );
    return d.promise();
  };

}();

pipe takes functions, not Promises. Try changing your prepareExtra to:

var prepareExtra = function( div ) {
  return function () {
      return $.post( div.attr( "href" ), {}, function( data ) {
        div.html( data )
      } );
  };
};

So that it returns a function that will be called when the previous Deferred is resolved.